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 <ro.ok@me.com>

libs/libwebsockets/Makefile

@@ -21,6 +21,8 @@ PKG_SOURCE_VERSION:=v$(PKG_VERSION)
 PKG_LICENSE:=MIT
 PKG_LICENSE_FILES:=LICENSE
 
+PKG_BUILD_DEPENDS:=libubox
+
 CMAKE_INSTALL:=1
 
 include $(INCLUDE_DIR)/package.mk