cmake: Use correct input pointer type for iconv detection

The iconv function expects a char ** pointer argument for the input
pointer.  Future C compilers will flag this as an error (as it is not
valid C), causing this probe to fail incorrectly.

cmake/Modules/FindIconv.cmake

@@ -36,7 +36,7 @@ IF(ICONV_FOUND)
   #include <iconv.h>
   int main(){
     iconv_t conv = 0;
-    const char* in = 0;
+    char* in = 0;
     size_t ilen = 0;
     char* out = 0;
     size_t olen = 0;