cmake: explicitly enable int-conversion warnings

While GCC enables int-conversion warnings by default, it will currently
only warn about such errors even in case where "-DENABLE_WERROR=ON" has
been passed to CMake. Explicitly enable int-conversion warnings by using
our `ENABLE_WARNINGS` macro, which will automatically use
"-Werror=int-conversions" in case it has been requested by the user.

(cherry picked from commit aa0ae59a)

CMakeLists.txt

@@ -227,6 +227,7 @@ ELSE ()
 	DISABLE_WARNINGS(unused-function)
 	ENABLE_WARNINGS(format)
 	ENABLE_WARNINGS(format-security)
+	ENABLE_WARNINGS(int-conversion)
 
 	IF (APPLE) # Apple deprecated OpenSSL
 	    DISABLE_WARNINGS(deprecated-declarations)