testsuite: Fix gdb.arch/powerpc-prologue.c compilation

testsuite: Fix gdb.arch/powerpc-prologue.c compilation

gcc-6.2.1

gdb compile failed, gdb/testsuite/gdb.arch/powerpc-prologue.c: In function 'main':
gdb/testsuite/gdb.arch/powerpc-prologue.c:32:3: warning: implicit declaration of function 'optimized_1' [-Wimplicit-function-declaration]
   optimized_1 ();
   ^~~~~~~~~~~

gdb/testsuite/ChangeLog
2016-10-11  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.arch/powerpc-prologue.c (optimized_1): New declaration.