uml: fix glibc-static check with GCC14

uml: fix glibc-static check with GCC14

Running the glibc-static check with GCC14 as the host compiler will fail:
Please install a static glibc package. (Missing libutil.a, librt.a or libpthread.a)

However, this error will get printed even with the required static
libraries installed when GCC14 is used.

Manually running the check exposes the real error:
<stdin>: In function ‘main’:
<stdin>:1:45: error: implicit declaration of function ‘timer_gettime’ [-Wimplicit-function-declaration]

GCC14 now errors on implicit declarations by default, so lets add the
required time.h header to fix compilation and thus the check.

Link: https://github.com/openwrt/openwrt/pull/15778
Signed-off-by: Robert Marko <robimarko@gmail.com>