Fix pedantic warnings for gcc

Bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64502

Author: oxr463

sds.c

@@ -1266,6 +1266,7 @@ int sdsTest(void) {
                 if (type != SDS_TYPE_5) {
                     test_cond("sdsMakeRoomFor() free", sdsavail(x) >= step);
                     oldfree = sdsavail(x);
+                    (void) oldfree;
                 }
                 p = x+oldlen;
                 for (j = 0; j < step; j++) {

sds.h

@@ -272,3 +272,8 @@ int sdsTest(int argc, char *argv[]);
 #endif
 
 #endif
+
+/* GCC:  Incorrect warning about empty translation units
+ * when using pre-compiled headers,
+ * (See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64502). */
+typedef int sdsvoid;