cmake: fix compilation w/Xcode generator
cmake: fix compilation w/Xcode generator

Add a stub file to object only library targets.

https://cmake.org/cmake/help/v3.18/command/add_library.html#object-libraries
Some native build systems (such as Xcode) may not like targets that have
only object files, so consider adding at least one real source file to
any target that references $<TARGET_OBJECTS:objlib>.

Bug: webp:477
Change-Id: I5d404c921d84433c7a0b22e0736ab0dbe699d264

webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp