run-command.c: fix missing include under `NO_PTHREADS`

Git 2.41-rc0 fails to compile run-command.c with `NO_PTHREADS` defined,
i.e.

  $ make NO_PTHREADS=1 run-command.o

as `ALLOC_GROW()` macro is used in the `atexit()` emulation but the
macro definition is not available.

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()`).

We can see that run-command.c is the only one that try to use these
macros without including <alloc.h>.

  $ git grep -l -e '[^_]ALLOC_GROW(' -e 'ALLOC_GROW_BY(' \*.c | sort >/tmp/1
  $ git grep -l 'alloc\.h' \*.c | sort >/tmp/2
  $ comm -23 /tmp/[12]
  compat/simple-ipc/ipc-unix-socket.c
  run-command.c

The "compat/" file only talks about the macro in the comment,
and is not broken.

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 <randall.becker@nexbridge.ca>
Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -11,6 +11,7 @@
 #include "packfile.h"
 #include "hook.h"
 #include "compat/nonblock.h"
+#include "alloc.h"
 
 void child_process_init(struct child_process *child)
 {