Forward declare stdio.h function snprintf.

radixgeo/coordinateconversion.cc

@@ -5,6 +5,9 @@
 #include <stdio.h>
 #include <algorithm>
 
+// forward declaration of stdio.h function
+int snprintf(char* buf, size_t size, const char* fmt, ...);
+
 namespace radix
 {
 const std::array<char, 22> UTMZones::letters = {