entrypoint: remove extern "C" from main declaration

clang emits -Wmain if `main` is declared as `extern "C"`.

Pick-to: 6.8
Change-Id: Ieee28204f18e5153eaae8d6088294539f4991f96
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 6d820c85510ad440a444bb0aba73b8ad887d9251)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/entrypoint/qtentrypoint_win.cpp

@@ -18,7 +18,7 @@
 int qMain(int, char **);
 #define main qMain
 #else
-extern "C" int main(int, char **);
+int main(int, char **);
 #endif
 
 /*