Reduce <signal.h> namespace pollution.

<signal.h> shouldn't drag in all of <string.h>.

Change-Id: I57aea36dad5e89ea2f9541c35594ca6225e532be

libc/bionic/sigemptyset.cpp

@@ -28,6 +28,7 @@
 
 #include <errno.h>
 #include <signal.h>
+#include <string.h>
 
 int sigemptyset(sigset_t* set) {
   if (set == NULL) {

libc/bionic/sigfillset.cpp

@@ -28,6 +28,7 @@
 
 #include <errno.h>
 #include <signal.h>
+#include <string.h>
 
 int sigfillset(sigset_t* set) {
   if (set == NULL) {

libc/include/signal.h

@@ -33,7 +33,6 @@
 #include <limits.h>
 #include <machine/pthread_types.h>
 #include <machine/timespec.h>
-#include <string.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>