ament_add_gtest_executable: Uses old style target_link_libraries, should use keyword if possible?

[EricCousineau-TRI]

In relation to comment here:
ros2/rosidl#400 (comment)

I found that the macro ament_add_gtest_executable seems to not use the newer keyword style of linking:

ament_cmake/ament_cmake_gtest/cmake/ament_add_gtest_executable.cmake

Lines 52 to 54 in 799183a

	if(NOT ARG_SKIP_LINKING_MAIN_LIBRARIES)
	target_link_libraries("${target}" ${GTEST_MAIN_LIBRARIES})
	endif()

Relates #383

\cc @sloretz @IanTheEngineer

[sloretz]

I found that the macro ament_add_gtest_executable seems to not use the newer keyword style of linking:

IIUC the keyword argument probably doesn't matter for executable targets since no downstream target is going to link against it, but CMake probably still requires it to be all keyword based or all plain (CMP0023). We could change this to PRIVATE on Rolling so it works with macros that pass a keyword argument to target_link_libraries().