powerpc/lib/code-patching: Make instr_is_branch_to_addr() static

instr_is_branch_to_addr() is only used in code-patching.c

Make it static.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5f6b9c8c83170ed310953eac2f5b14539bfc964a.1621516826.git.christophe.leroy@csgroup.eu