gdb: fix include for gdb_signal in target/waitstatus.h

clangd tells me that the gdb_signals.h include in target/waitstatus.h is
unused.  This include was probably to give access to `enum gdb_signal`,
but this is in fact defined in gdb/signals.h.  Change the include to
gdb/signals.h.  Include gdbsupport/gdb_signals.h in some files that were
relying on the transitive include.

Change-Id: I6f4361b3d801394bf29abe8c1393aff110aa0ad6

gdb/nat/fork-inferior.c

@@ -26,6 +26,7 @@
 #include "gdbsupport/pathstuff.h"
 #include "gdbsupport/signals-state-save-restore.h"
 #include "gdbsupport/gdb_tilde_expand.h"
+#include "gdbsupport/gdb_signals.h"
 #include <vector>
 
 extern char **environ;

gdb/target/waitstatus.c

@@ -18,6 +18,7 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "waitstatus.h"
+#include "gdbsupport/gdb_signals.h"
 
 /* See waitstatus.h.  */
 

gdb/target/waitstatus.h

@@ -21,7 +21,7 @@
 #define TARGET_WAITSTATUS_H
 
 #include "diagnostics.h"
-#include "gdbsupport/gdb_signals.h"
+#include "gdb/signals.h"
 
 /* Stuff for target_wait.  */