fix missing prototype for strsignal

include/string.h

@@ -56,6 +56,7 @@ char *stpncpy(char *, const char *, size_t);
 size_t strnlen (const char *, size_t);
 char *strdup (const char *);
 char *strndup (const char *, size_t);
+char *strsignal(int);
 #endif
 
 #if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE)

src/string/strsignal.c

@@ -1,4 +1,5 @@
 #include <signal.h>
+#include <string.h>
 
 #if (SIGHUP == 1) && (SIGINT == 2) && (SIGQUIT == 3) && (SIGILL == 4) \
  && (SIGTRAP == 5) && (SIGABRT == 6) && (SIGBUS == 7) && (SIGFPE == 8) \