[build] Explicitly tell CMake that the project uses C language

This fixes a CMake error:

```
CheckSourceCompiles.cmake:44 (message):
  check_source_compiles: C: needs to be enabled before use.
```

Author: egorzhdan

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.18)
 
-project(SwiftLLVMBindings LANGUAGES Swift)
+project(SwiftLLVMBindings LANGUAGES C Swift)
 
 if(CMAKE_SYSTEM_NAME STREQUAL Windows OR CMAKE_SYSTEM_NAME STREQUAL Darwin)
   option(BUILD_SHARED_LIBS "Build shared libraries by default" YES)