sparc: fix build of gdb/testsuite/gdb.arch/sparc-sysstep.c

This patch adds a missing include that makes the test program to not
be built (--Wimplicit-function-declaration).

gdb/testsuite/ChangeLog:

2015-11-20  Jose E. Marchesi  <jose.marchesi@oracle.com>

    	* gdb.arch/sparc-sysstep.c: Include unistd.h for getpid.