Build intermediate EtcLib always as STATIC

Depending on the BUILD_SHARED_LIBS variable EtcLib may be built as SHARED
if not set explicitly, like done for the other libraries in extern.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

extern/EtcLib/CMakeLists.txt

@@ -21,4 +21,4 @@ file(GLOB SOURCES
 	${PROJECT_SOURCE_DIR}/EtcCodec/*.h
 	${PROJECT_SOURCE_DIR}/Etc/*.cpp
 	${PROJECT_SOURCE_DIR}/EtcCodec/*.cpp)
-ADD_LIBRARY(EtcLib ${SOURCES})
+ADD_LIBRARY(EtcLib STATIC ${SOURCES})