Fix flag check for fuzzers (halide#7542)

On some system size_t isn't available under <cstdint>, however
it is garaunteed to be available under <cstddef> for all systems.

test/CMakeLists.txt

@@ -75,6 +75,7 @@ set(CMAKE_REQUIRED_LINK_OPTIONS "-fsanitize=fuzzer,address")
 set(CMAKE_REQUIRED_FLAGS "-fsanitize=fuzzer-no-link,address")
 check_cxx_source_compiles([[
 #include <cstdint>
+#include <cstddef>
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, std::size_t Size) {
   return 0;
 }