cmake: Do not install static libraries in shared build mode.

cmake: Do not install static libraries in shared build mode.

The glslang cmake build system installs static libraries in addition to
the dynamic glslang library, which are required when used in a package
that uses glslang when calling find_package().

Distributions such as openSUSE (and perhaps others) use a "shared only"
strategy, which conflicts with the current state of the glslang build
system.
The static libraries mentioned are already all included in glslang and
thus not needed. With this commit, these will no longer install the
associated cmake support files when glslang is built shared.