Use idomatic approach to extending CMAKE_MODULE_PATH. (#7)

There are many reasons why there may already be a CMAKE_MODULE_PATH defined.
The idiomatic way to extend it in a project is via list(APPEND).

https://github.com/ROCm/rocminfo/pull/97 submitted by @stellaraccident

Co-authored-by: Stella Laurenzo <Stella.Laurenzo@amd.com>

CMakeLists.txt

@@ -55,9 +55,7 @@ endif()
 # Default to ON
 option(BUILD_SHARED_LIBS "Build using shared libraries" ON)
 ## Set default module path if not already set
-if(NOT DEFINED CMAKE_MODULE_PATH)
-    set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake_modules/")
-endif()
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake_modules/")
 ## Include common cmake modules
 include(utils)