libcompat.h: strsignal() should be not be decleared const

POSIX standards requires strsignal() to return a pointer to a char,
not a const pointer to a char. [1]  On uClibc, and possibly other
libc's, that do not HAVE_DECL_STRSIGNAL, libcompat.h declares
const char *strsignal (int sig) which causes a type error.

[1] man 3 strsignal

https://bugzilla.gnome.org/show_bug.cgi?id=763567

libs/gst/check/libcheck/strsignal.c

@@ -1,6 +1,6 @@
 #include "libcompat.h"
 
-const char *
+char *
 strsignal (int sig)
 {
   static char signame[40];