cmake: do not set CMAKE_SKIP_INSTALL_RULES to ON

This CMakeLists.txt is added via add_subdirectory() to the app's
CMakeLists.txt. If the app-CMakeLists.txt has install() rules and
wants to use them, setting CMAKE_SKIP_INSTALL_RULES to ON breaks
the 'make install' call.

Not setting this variable does not harm zephyr because zephyr does
not use install().

Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>