Make sure to find_package(rmw) in rmw_implementation. (#242) (#245)

This is required to get access to the register_rmw_implementation
CMake macro.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>
(cherry picked from commit e9f6008)

Co-authored-by: Chris Lalancette <clalancette@gmail.com>

Author: mergify[bot]

rmw_implementation/CMakeLists.txt

@@ -13,6 +13,7 @@ if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 endif()
 
 find_package(ament_cmake REQUIRED)
+find_package(rmw REQUIRED)
 find_package(rmw_implementation_cmake REQUIRED)
 
 if(BUILD_TESTING)