fuzz target: add #if guard for extern "C"

regress/zip_read_fuzzer.cc

diff --git a/regress/zip_read_fuzzer.cc b/regress/zip_read_fuzzer.cc
index 9128dca..eaa42dd 100644
--- a/regress/zip_read_fuzzer.cc
+++ b/regress/zip_read_fuzzer.cc
@@ -1,6 +1,9 @@
 #include "../lib/zip.h"
 
-extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
+#ifdef __cplusplus
+extern "C"
+#endif
+int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
     zip_source_t *src;
     zip_t *za;