Fix cmake error when build with capstone as submodule (#2619)

Author: WerWolv

CPackConfig.txt

@@ -9,7 +9,7 @@ set(CPACK_PACKAGE_DESCRIPTION "Capstone is a lightweight multi-platform, multi-a
 set(CPACK_PACKAGE_DESCRIPTION_SUMMARY "Lightweight multi-architecture disassembly framework - devel files")
 set(CPACK_PACKAGE_HOMEPAGE_URL "https://www.capstone-engine.org/")
 set(CPACK_STRIP_FILES false)
-set(CPACK_RESOURCE_FILE_LICENSE "${CMAKE_SOURCE_DIR}/LICENSE.TXT")
+set(CPACK_RESOURCE_FILE_LICENSE "${CMAKE_CURRENT_SOURCE_DIR}/LICENSE.TXT")
 
 # Set Debian-specific package variables
 set(CPACK_DEBIAN_PACKAGE_NAME "libcapstone-dev")