Fix SDL2 pkg-config name

The pkg-config name for SDL 2 is lowercase 'sdl2', and it was passed in
upper-case so the proper library was never detected on Linux. Fix the name.

lib/SDL2.cmake

@@ -53,7 +53,7 @@ else()
         # CMake can't check for SDL2 so we'll just use PkgConfig
         find_package(PkgConfig)
 
-        PKG_SEARCH_MODULE(SDL2 "SDL2 >= 2.26.5")
+        PKG_SEARCH_MODULE(SDL2 "sdl2 >= 2.26.5")
         if(SDL2_FOUND)
             PKG_CONFIG_LIB_RESOLVE(SDL2 SDL2_LIB)
             ADD_IMPORTED_LIB(sdl2 "${SDL2_INCLUDE_DIRS}" "${SDL2_LIB}")
@@ -78,4 +78,4 @@ else()
         file(GLOB SDL2_LIBS "${SDL2_ROOT_DIR}/lib/libSDL2-2*")
         add_target_copy_files("${SDL2_LIBS}")
     endif()
-endif()
+endif()