Add wxGCD() helper function.

It is needed in wxMSW code and it looks like it could be useful to the library
users, so make it public.

See #16254.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77019 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/math.h

@@ -16,6 +16,15 @@
 */
 int wxFinite(double x);
 
+/**
+    Returns the greatest common divisor of the two given numbers.
+
+    @since 3.1.0
+
+    @header{wx/math.h}
+*/
+unsigned int wxGCD(unsigned int u, unsigned int v);
+
 /**
     Returns a non-zero value if x is NaN (not a number), returns 0 otherwise.