dlfcn: Eliminate GLIBC_PRIVATE dependency from tststatic2

dlfcn: Eliminate GLIBC_PRIVATE dependency from tststatic2

The test appears to use _dlfcn_hook@@GLIBC_PRIVATE as a way to
test dlvsym without having to know the appropriate symbol version.
With <first-versions.h>, we can use a public symbol and the symbol
version at which it was defined first.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>