shared: struct timespec is in time.h

On the musl C library, tests/timespec-text.c does not build, with the
following error:

    In file included from tests/timespec-test.c:36:0:
    ./shared/timespec-util.h:41:21: warning: ‘struct timespec’ declared
    inside parameter list will not be visible outside of this definition
    or declaration
     timespec_sub(struct timespec *r,
                         ^~~~~~~~
   [...]

Indeed, struct timespec is defined in time.h, so we must include it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

shared/timespec-util.h

@@ -28,6 +28,7 @@
 
 #include <stdint.h>
 #include <assert.h>
+#include <time.h>
 
 #define NSEC_PER_SEC 1000000000