CMake: make option ENABLE_SDL work again

Broken in PR #4755 / commit f978765

Source/Core/InputCommon/CMakeLists.txt

@@ -51,6 +51,7 @@ if(UNIX)
 	set(SRCS ${SRCS} ControllerInterface/Pipes/Pipes.cpp)
 endif()
 
+if(ENABLE_SDL)
 	find_package(SDL2)
 	if(SDL2_FOUND)
 		message(STATUS "Using shared SDL2")
@@ -73,5 +74,6 @@ if(SDL_TARGET AND TARGET ${SDL_TARGET})
 	else()
 	  message(STATUS "SDL NOT found, disabling SDL input")
 	endif()
+endif()
 
 add_dolphin_library(inputcommon "${SRCS}" "${LIBS}")