uqm: Fix build against glibc 2.26

I've bisected the introduction of the build failure to be the glibc 2.26
upgrade with commit 9bb67d5c.

At first I was somewhat stumped why an glibc update could cause
undeclared identifiers, but after looking at the changes of glibc and
the source code of The Ur-Quan Masters the problem quickly turned out to
be this very change:

https://sourceware.org/git/?p=glibc.git;a=commit;h=7b037c095e31c2396d0a9b0e6356bc566ee4812f



So string.h now in turn includes strings.h, which in theory wouldn't be
a problem.

However, both strings.h and the strings.h in the uqm source code use
constant _STRINGS_H, which causes the glibc strings.h to be included but
the one from uqm basically includes an empty file.

Signed-off-by: aszlig <aszlig@nix.build>