Fixes the following testsuite failure:

$ sudo ./perf test -v kallsyms
 1: vmlinux symtab matches kallsyms                          :

--- start ---

test child forked, pid 12489
Using /proc/kcore for kernel object code
Looking at the vmlinux_path (8 entries long)
Using /boot/vmlinux for symbols
0xc00000000003d300: diff name v: .kretprobe_trampoline_holder k: kretprobe_trampoline
Maps only in vmlinux:
 c00000000086ca38-c000000000879b6c 87ca38 [kernel].text.unlikely
 c000000000879b6c-c000000000bf0000 889b6c [kernel].meminit.text
 c000000000bf0000-c000000000c53264 c00000 [kernel].init.text
 c000000000c53264-d000000004250000 c63264 [kernel].exit.text
 d000000004250000-d000000004450000 0 [libcrc32c]
 d000000004450000-d000000004620000 0 [xfs]
 d000000004620000-d000000004680000 0 [autofs4]
 d000000004680000-d0000000046e0000 0 [x_tables]
 d0000000046e0000-d000000004780000 0 [ip_tables]
 d000000004780000-d0000000047e0000 0 [rng_core]
 d0000000047e0000-ffffffffffffffff 0 [pseries_rng]
Maps in vmlinux with a different name in kallsyms:
Maps only in kallsyms:
 d000000000000000-f000000000000000 1000000000010000 [kernel.kallsyms]
 f000000000000000-ffffffffffffffff 3000000000010000 [kernel.kallsyms]
test child finished with -1
---- end ----
vmlinux symtab matches kallsyms: FAILED!

The problem is that the kretprobe_trampoline symbol looks like this:

$ eu-readelf -s /boot/vmlinux G kretprobe_trampoline
 2431: c000000001302368     24 NOTYPE  LOCAL  DEFAULT       37 kretprobe_trampoline_holder
 2432: c00000000003d300      8 FUNC    LOCAL  DEFAULT        1 .kretprobe_trampoline_holder
97543: c00000000003d300      0 NOTYPE  GLOBAL DEFAULT        1 kretprobe_trampoline

Its type is NOTYPE, and its size is 0, and this is a problem because
symbol-elf.c:dso__load_sym skips function symbols that are not STT_FUNC
or STT_GNU_IFUNC (this is determined by elf_sym__is_function). Even
if the type is changed to STT_FUNC, when dso__load_sym calls
symbols__fixup_duplicate, the kretprobe_trampoline symbol is dropped in
favour of .kretprobe_trampoline_holder because the latter has non-zero
size (as determined by choose_best_symbol).

With this patch, during symbol deduplication kretprobe_trampoline is
used instead of .kretprobe_trampoline_holder, making all vmlinux symbols match
/proc/kallsyms and the testcase passes.