Wrap ruby_abi_version in `extern "C"` for C++

Make ruby_abi_version have C linkage so that the symbol can be found
in the shared object.

include/ruby/internal/abi.h

@@ -32,12 +32,20 @@
 
 #ifdef RUBY_DLN_CHECK_ABI
 
+# ifdef __cplusplus
+extern "C" {
+# endif
+
 RUBY_FUNC_EXPORTED unsigned long long __attribute__((weak))
 ruby_abi_version(void)
 {
     return RUBY_ABI_VERSION;
 }
 
+# ifdef __cplusplus
+}
+# endif
+
 #endif
 
 #endif