Fix warning

Not sure what the story is with this file, but it's making a
warning and this fixes it.

src/crypto/crc.cpp

@@ -615,6 +615,8 @@ uint64_t _mm_crc32_u64(uint64_t a, uint64_t b )
 
 int main( int argc, char** argv )
 {
+    // Squelch warning about unusued variable crc_c
+    (void)(crc_c);
     uint64_t f = 0x1234;
     uint64_t a = 0x5678;
     uint32_t f1 = _mm_crc32_u64(f, a);