applets/clock: Fix compatibility with musl libc

[sgn]

%l is an glibc extension for strftime, this extensions isn't supported by musl libc.

Replace %l with %_I, which is functionaly equivalence. %_I itself is also an extensions, however, %_I is supported by both glibc, musl, FreeBSD and deriviates, AIX, and Solaris.

Fix: #1451

[lukefromdc]

I cannot test this except for breaking changes with glibc, we don't have musl libc on Debian Unstable

[sgn]

Does it break for glibc? We have musl over here, and it seems to work correctly.

[lukefromdc]

I can test under libglibc when I get home