gdb: include gdb_wait.h in gdb_wait.c

When building for mingw with -Wmissing-declarations, I get:

  CXX    gdbsupport/gdb_wait.o
/binutils-gdb/gdb/gdbsupport/gdb_wait.c:52:1: error: no previous declaration for 'int windows_status_to_termsig(long unsigned int)' [-Wer
ror=missing-declarations]
   52 | windows_status_to_termsig (unsigned long status)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~

Make gdb_wait.c include gdb_wait.h to fix it.

gdb/ChangeLog:

	* gdbsupport/gdb_wait.c: Include gdb_wait.h.

gdb/ChangeLog

+2020-01-12  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* gdbsupport/gdb_wait.c: Include gdb_wait.h.
+
 2020-01-12  Pedro Alves  <palves@redhat.com>
 
 	* bsd-kvm.c (bsd_kvm_target::close): Call exit_inferior_silent

gdb/gdbsupport/gdb_wait.c

@@ -19,6 +19,8 @@
 
 #include "common-defs.h"
 
+#include "gdb_wait.h"
+
 #ifdef __MINGW32__
 
 /* The underlying idea is that when a Windows program is terminated by