powerpc/time: Force inlining of get_tb()

Force inlining of get_tb() in order to avoid getting
following function in vdso32, leading to suboptimal
performance in clock_gettime()

00000688 <.get_tb>:
 688:	7c 6d 42 a6 	mftbu   r3
 68c:	7c 8c 42 a6 	mftb    r4
 690:	7d 2d 42 a6 	mftbu   r9
 694:	7c 03 48 40 	cmplw   r3,r9
 698:	40 e2 ff f0 	bne+    688 <.get_tb>
 69c:	4e 80 00 20 	blr

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/df05d53eed1210cf1aa76d1fb44aa0fab29c018e.1608488286.git.christophe.leroy@csgroup.eu