[PATCH v3 2/4] compat/regex: move stdlib.h up in inclusion chain

In Linux with musl libc, we have this inclusion chain:

compat/regex/regex.c:69
`-> compat/regex/regex_internal.h
   `-> /usr/include/stdlib.h
      `-> /usr/include/features.h
      `-> /usr/include/alloca.h

In that inclusion chain, `<features.h>` claims it's _BSD_SOURCE compatible when it's NOT asked to be either {_POSIX,_GNU,_XOPEN,_BSD}_SOURCE, or __STRICT_ANSI__. And, `<stdlib.h>` will include `<alloca.h>` to be compatible with software written for GNU and BSD. Thus, redefine `alloca` macro, which was defined before at compat/regex/regex.c:66.

Considering this is only compat code, we've taken from other project, it's not our business to decide which source should we adhere to.

Include `<stdlib.h>` early to prevent the redefinition of alloca.
This also remove a potential warning about alloca not defined on:
	#undef alloca

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
---

Notes:
    There is another warning about un-match declaration and definition of
    re_string_context_at.
    
    It's arguably, a bug in sparse itself.
    Consider that there's a fix for it in their development version already,
    and, we expect they'll fix the zero-ing pattern in 1/? before enable it again.
    
    There're no point to change that pair of declaration/definition.

    This patch is technically Ramsay's work.
    Since <alloca.h> is likely unportable, my patch will be likely unusable.
    I took Ramsay's work and write the commit message instead.

    I would love to see Ramsay's SoB on reply to this patch.

 compat/regex/regex.c          | 1 +
 compat/regex/regex_internal.h | 1 -
 2 files changed, 1 insertion(+), 1 deletion(-)