When building git with `NO_PTHREADS=YesPlease`, we fail to build
run-command.o since we don't have a definition for ALLOC_GROW:

    $ make NO_PTHREADS=1 DEVELOPER=1 run-command.o
    GIT_VERSION = 2.41.0.rc0.1.g787eb3beae.dirty
        CC run-command.o
    run-command.c: In function ‘git_atexit’:
    run-command.c:1103:9: error: implicit declaration of function ‘ALLOC_GROW’ [-Werror=implicit-function-declaration]
     1103 |         ALLOC_GROW(git_atexit_hdlrs.handlers, git_atexit_hdlrs.nr + 1, git_atexit_hdlrs.alloc);
          |         ^~~~~~~~~~
    cc1: all warnings being treated as errors
    make: *** [Makefile:2715: run-command.o] Error 1

This bisects to 36bf195890 (alloc.h: move ALLOC_GROW() functions from
cache.h, 2023-02-24), which replaced includes of "cache.h" with
"alloc.h", which is the new home of `ALLOC_GROW()` (and
`ALLOC_GROW_BY()`).

run-command.c compiles fine when `NO_PTHREADS` is undefined, since its
use of `ALLOC_GROW()` is behind a `#ifndef NO_PTHREADS`. (Everything
else compiles fine when NO_PTHREADS is defined, so this is the only spot
that needs fixing).

We could fix this by conditionally including "alloc.h" when
`NO_PTHREADS` is defined.  But we have relatively few examples of
conditional includes throughout the tree[^1].

Instead, include "alloc.h" unconditionally in run-command.c to allow it
to successfully compile even when NO_PTHREADS is defined.

[^1]: With `git grep -E -A1 '#if(n)?def' -- **/*.c | grep '#include' -B1`.

Reported-by: Randall S. Becker <redacted>
Signed-off-by: Taylor Blau <redacted>
---
 run-command.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/run-command.c b/run-command.c
index d4247d5fcc..60c9419866 100644
--- a/run-command.c
+++ b/run-command.c

@@ -16,6 +16,7 @@
 #include "packfile.h"
 #include "hook.h"
 #include "compat/nonblock.h"
+#include "alloc.h"
 
 void child_process_init(struct child_process *child)
 {

-- 
2.41.0.rc0.dirty