if __NetBSD__ defined, don't prototype strsignal() -- string.h already

provides a prototype, and it's different from what was here!
(proto here was const char *..., proto in string.h was just char *...)

gnu/usr.bin/groff/groff/groff.cc

@@ -50,7 +50,10 @@ extern "C" {
 }
 #endif /* not STDLIB_H_DECLARES_PUTENV */
 
+#ifndef __NetBSD__
+/* defined in <string.h> in NetBSD */
 const char *strsignal(int);
+#endif
 
 const int SOELIM_INDEX = 0;
 const int REFER_INDEX = SOELIM_INDEX + 1;