Include required module before using check_c_source_compiles()

Fix CMake build with 3.10, which gave the following errors

CMake Error at build/cmake/init.cmake:321 (check_c_source_compiles):
  Unknown CMake command "check_c_source_compiles".
Call Stack (most recent call first):
  build/cmake/main.cmake:16 (include)
  CMakeLists.txt:69 (include)

previously. Apparently the required CheckCSourceCompiles module was
already included from somewhere else with later CMake versions, but not
with this one, so do include it explicitly.

build/cmake/init.cmake

@@ -317,6 +317,7 @@ if (wxUSE_WEBREQUEST)
         endif()
     endif()
 
+    include(CheckCSourceCompiles)
     if(wxUSE_WEBREQUEST_WINHTTP)
         check_c_source_compiles("#include <windows.h>
                                  #include <winhttp.h>