libctf: Add configure check for asprintf (for MinGW)

libctf: Add configure check for asprintf (for MinGW)

This commit fixes a compilation warning when compiling libctf
on MinGW:

    libctf/ctf-dump.c:118:8: warning: implicit declaration of function
    'asprintf'; did you mean 'vasprintf'? [-Wimplicit-function-declaration]

	 if (asprintf (&bit, " %lx: [slice 0x%x:0x%x]",
	     ^~~~~~~~
	     vasprintf

MinGW doesn't provide that function, so we depend on the one provided
by libiberty. However, the declaration is guarded by HAVE_DECL_ASPRINTF,
which we do not have in libctf's config.h.

libctf/ChangeLog:

	PR binutils/25155:
	* configure.ac: Add AC_CHECK_DECLS([asprintf]).
	* configure, config.h.in: Regenerate.