Make tst-strftime2 and tst-strftime3 depend on locale generation

Building the test cases in parallel might make tst-strftime2 and
tst-strftime3 fail.  Simply re-running the test case (or building
serially) makes the problem go away.  This patch adds the necessary
dependency to allow parallel builds in the time subdirectory.

Tested for powerpc64le.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>