Fix header name and move return outside of ifdef.

Fixes from Mike Frysinger via Github PR#597.

configure.ac

@@ -536,7 +536,7 @@ AC_CHECK_HEADERS([ \
     nlist.h \
     poll.h \
     stdint.h \
-    sys/mmap.h \
+    sys/mman.h \
     sys/stat.h \
     sys/time.h \
     sys/un.h \

sshkey.c

@@ -734,10 +734,10 @@ sshkey_prekey_alloc(u_char **prekeyp, size_t len)
 	(void)madvise(prekey, len, MADV_DONTDUMP);
 #endif
 	*prekeyp = prekey;
-	return 0;
 #else
 	*prekeyp = calloc(1, len);
 #endif /* HAVE_MMAP et al */
+	return 0;
 }
 
 static void