[fuzzer] Fix build on musl

cstddef is needed for size_t definition.
(Multiple headers can provide size_t but none of them exists.)

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D96213

compiler-rt/lib/fuzzer/FuzzerInterceptors.cpp

@@ -25,6 +25,7 @@
   }
 
 #include <cassert>
+#include <cstddef> // for size_t
 #include <cstdint>
 #include <dlfcn.h> // for dlsym()