eudev: fix mkstemp/mkostemp related build failure

Fixes [1]:

./.libs/libudev-core.a(util.o): In function `mkostemp_safe':
util.c:(.text+0x2950): undefined reference to `mkostemp'

Do not only protect the call to the mkostemp wrapper mkostemp_safe
with HAVE_DECL_MKOSTEMP but also the wrapper itself.

[1] http://autobuild.buildroot.net/results/fa6/fa62883de4ada664521ffc47b611ecd2279c0014/



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>