MinGW compilation warnings in libiberty's waitpid.c

[Eli Zaretskii <eliz@gnu.org>]

When compiling libiberty (as part of GDB) with MinGW on MS-Windows, I
see the following warning:

     gcc -c -DHAVE_CONFIG_H -O2 -gdwarf-4 -g3 -D__USE_MINGW_ACCESS  -I. -I./../include   -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic  -D_GNU_SOURCE ./waitpid.c -o waitpid.o
     ./waitpid.c: In function 'waitpid':
     ./waitpid.c:31:18: warning: implicit declaration of function 'wait' [-Wimplicit-function-declaration]
	    int wpid = wait(stat_loc);
		       ^

The file waitpid.c should not be built on MinGW, as it is not needed
on Windows, and will not work if the function is called (because
there's no 'wait' function on MS-Windows).