Avoid redundant redeclaration of GetWarnings(const string&)

std::string GetWarnings(const std::string& strFor) is declared in
warnings.h and defined in warnings.cpp.

src/qt/clientmodel.cpp

@@ -18,6 +18,7 @@
 #include "txmempool.h"
 #include "ui_interface.h"
 #include "util.h"
+#include "warnings.h"
 
 #include <stdint.h>