build: fix CMake generator for non-Ninja builds

build: fix CMake generator for non-Ninja builds

OpenWRT by default uses the Ninja generator, but some packages disable
Ninja and use the default Unix Makefiles generator. This generator can
be overridden in the user environment with `CMAKE_GENERATOR`. This patch
explicitly sets the correct generator when `PKG_USE_NINJA:=0`.

In particular, the `mt76` package uses the Makefiles generator.

Signed-off-by: Ivan Romanov <drizt72@zoho.eu>
Link: https://github.com/openwrt/openwrt/pull/16263
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>