Fix the include path in the installed CMake target import file

Previously the include path was {INSTALL_PREFIX}/include,
it is now {INSTALL_PREFIX}/include/SDL2 to be consistent with
the other build and package configuration systems.

Fixes #4128.

CMakeLists.txt

@@ -1695,7 +1695,7 @@ set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${EXTRA_CFLAGS}")
 
 # Always build SDLmain
 add_library(SDL2main STATIC ${SDLMAIN_SOURCES})
-target_include_directories(SDL2main PUBLIC $<INSTALL_INTERFACE:include>)
+target_include_directories(SDL2main PUBLIC $<INSTALL_INTERFACE:include/SDL2>)
 set(_INSTALL_LIBS "SDL2main")
 if (NOT ANDROID)
   set_target_properties(SDL2main PROPERTIES DEBUG_POSTFIX ${SDL_CMAKE_DEBUG_POSTFIX})
@@ -1724,7 +1724,7 @@ if(SDL_SHARED)
   endif()
   set(_INSTALL_LIBS "SDL2" ${_INSTALL_LIBS})
   target_link_libraries(SDL2 ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
-  target_include_directories(SDL2 PUBLIC $<INSTALL_INTERFACE:include>)
+  target_include_directories(SDL2 PUBLIC $<INSTALL_INTERFACE:include/SDL2>)
   if (NOT ANDROID)
     set_target_properties(SDL2 PROPERTIES DEBUG_POSTFIX ${SDL_CMAKE_DEBUG_POSTFIX})
   endif()
@@ -1750,7 +1750,7 @@ if(SDL_STATIC)
   # libraries - do we need to consider this?
   set(_INSTALL_LIBS "SDL2-static" ${_INSTALL_LIBS})
   target_link_libraries(SDL2-static ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
-  target_include_directories(SDL2-static PUBLIC $<INSTALL_INTERFACE:include>)
+  target_include_directories(SDL2-static PUBLIC $<INSTALL_INTERFACE:include/SDL2>)
   if (NOT ANDROID)
     set_target_properties(SDL2-static PROPERTIES DEBUG_POSTFIX ${SDL_CMAKE_DEBUG_POSTFIX})
   endif()