Fix test-skeleton C99 designed initialization

Fix test-skeleton C99 designed initialization

ISO C forbids empty initializer braces (6.7.9 initializer-list must
contain at least one initializer). However GCC allows it, generating
a warning depending of the version.

With GCC 4.8 on ARM I noticed tst-initializers1.c fails to build with:

In file included from tst-initializers1.c:60:0:
../test-skeleton.c: In function 'delayed_exit_thread':
../test-skeleton.c:687:10: error: missing initializer for field 'tv_sec' of 'struct timespec' [-Werror=missing-field-initializers]
   struct timespec remaining = {}

While with GCC 5.1 the same warning is just spilled with -pedantic.
To be safe this patch just zero initialize the struct as expected.

Tested on armhf.

	* test-skeleton.c (delayed_exit_thread): Add initializer on struct
	timespec C99 designated initialization.