system_win32: fix clang warning

- Declare variable in header as extern.

Bug: https://github.com/curl/curl/commit/48b9ea4#commitcomment-34084597

diff --git a/lib/system_win32.h b/lib/system_win32.h
index ef83a512cc5ddc501eb40db9660ad402081d2b75..d2882fce1ae524b468f8b619e095304b9e02e6c9 100644 (file)
--- a/lib/system_win32.h
+++ b/lib/system_win32.h
@@ -52,7 +52,7 @@ typedef enum {
 typedef unsigned int(WINAPI *IF_NAMETOINDEX_FN)(const char *);
 
 /* This is used instead of if_nametoindex if available on Windows */
-IF_NAMETOINDEX_FN Curl_if_nametoindex;
+extern IF_NAMETOINDEX_FN Curl_if_nametoindex;
 
 /* This is used to verify if we are running on a specific windows version */
 bool Curl_verify_windows_version(const unsigned int majorVersion,