Provide fake DllMain declaration to fix missing-declarations warning (#130)

src/dllmain.cpp

@@ -1,5 +1,7 @@
 #include <windows.h>
 
+// outsmart GCC's missing-declarations warning
+BOOL WINAPI DllMain(HMODULE, DWORD, LPVOID);
 BOOL WINAPI DllMain(HMODULE, DWORD, LPVOID)
 {
     return TRUE;