Move wxCTZ() documentation to the appropriate place

The documentation comment belongs to interface/wx/math.h, not the header
under include.

Also change the argument type to wxUint32 as the non-gcc version only
works for 32 bit values.

src/common/utilscmn.cpp

@@ -1021,7 +1021,7 @@ unsigned int wxGCD(unsigned int u, unsigned int v)
 // wxCTZ
 // Count trailing zeros. Use optimised builtin where available.
 // ----------------------------------------------------------------------------
-unsigned int wxCTZ(unsigned x)
+unsigned int wxCTZ(wxUint32 x)
 {
     wxCHECK_MSG(x > 0, 0, "Undefined for x == 0.");
 #ifdef __GNUC__