libwebsockets: avoid a compilation error due to a missing uloop

libwebsockets: avoid a compilation error due to a missing uloop

With a clean build environment and at least two parallel jobs
a compilation error may occur:

...
-- Looking for uv/version.h - found
libubox include dir: ULOOP_INCLUDE_DIRS-NOTFOUND
libubox libraries: ULOOP_LIBRARIES-NOTFOUND
...
CMake Error in lib/event-libs/uloop/CMakeLists.txt:
  Found relative path while evaluating include directories of
  "websockets-evlib_uloop":

    "ULOOP_INCLUDE_DIRS-NOTFOUND"
...

Signed-off-by: Ronny Kotzschmar <redacted>