tests: lib: c_lib: add the proper dependency for asctime_r and ctime_r

As the test uses reentrant time functions like asctime_r or ctime_r,
it should select POSIX_C_LANG_SUPPORT_R. Otherwise there are build
failures observed with the default test configuration if these
functions are not provided by the toolchain.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>