correct capitalization for libSDL2

Index: code/CMakeLists.txt
--- code/CMakeLists.txt.orig
+++ code/CMakeLists.txt
@@ -31,7 +31,7 @@ TARGET_LINK_LIBRARIES(code PUBLIC ${PNG_LIBS})
 TARGET_LINK_LIBRARIES(code PUBLIC ${JPEG_LIBS})
 TARGET_LINK_LIBRARIES(code PUBLIC lz4)
 
-TARGET_LINK_LIBRARIES(code PUBLIC sdl2)
+TARGET_LINK_LIBRARIES(code PUBLIC SDL2)
 
 if (FSO_BUILD_WITH_FFMPEG)
 	TARGET_LINK_LIBRARIES(code PUBLIC ffmpeg)
