toolchain/gdb: fix broken configure test for ELF support

A missing #include was causing gdb on macOS to be compiled without ELF support

Signed-off-by: Felix Fietkau <nbd@nbd.name>

toolchain/gdb/patches/100-fix-elf-support-check.patch

@@ -0,0 +1,20 @@
+--- a/gdb/configure
++++ b/gdb/configure
+@@ -16818,6 +16818,7 @@ else
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdlib.h>
++#include <string.h>
+   #include "bfd.h"
+   #include "elf-bfd.h"
+ 
+--- a/gdb/acinclude.m4
++++ b/gdb/acinclude.m4
+@@ -362,6 +362,7 @@ AC_DEFUN([GDB_AC_CHECK_BFD], [
+   AC_CACHE_CHECK([$1], [$2],
+   [AC_TRY_LINK(
+   [#include <stdlib.h>
++  #include <string.h>
+   #include "bfd.h"
+   #include "$4"
+   ],