meson: set the symbol visibility of the shared library to hidden

meson: set the symbol visibility of the shared library to hidden

This matches the Makefile build. Due to one private xxhash symbol in use
by the program, it recompiles a private copy of xxhash.

Due to the test binaries making extensive (?) use of private symbols, it
doesn't even attempt to link to shared libzstd, and instead, all of the
original object files are added to libtestcommon itself for private
linkage. This, too, matches the Makefile build.

Ref. #2261