libwebp/CMake: Add <BUILD_INTERFACE> to webp incl

This way target_link_libraries(my_target PUBLIC webp) is enough,
no need to add include directories explicitly for the public headers
such as src/webp/decode.h. This matches the /usr/include/webp/decode.h
installation folder path prefix (#include "webp/decode.h").

The target_include_directories() calls for cwebp, dwebp etc. may be
unnecessary now and may be removed in another patch.

Change-Id: I2c85218920ffc35ebec23e0dd239f71e29add23b
1 file changed