string: Fix prototype mismatch in sigabbrev_np, __sigdescr_np

GCC 6 does not ignore the const specifier on the return type.

string/sigabbrev_np.c

@@ -20,7 +20,7 @@
 #include <signal.h>
 #include <array_length.h>
 
-const char *const
+const char *
 sigabbrev_np (int signum)
 {
   const char *abbrev = NULL;

string/sigdescr_np.c

@@ -20,7 +20,7 @@
 #include <signal.h>
 #include <array_length.h>
 
-const char *const
+const char *
 __sigdescr_np (int signum)
 {
   const char *descr = NULL;