tests/bench: Rename test_akcipher_keys.inc -> test_akcipher_keys.c.inc

tests/bench: Rename test_akcipher_keys.inc -> test_akcipher_keys.c.inc

Since commit 139c1837db ("meson: rename included C source files
to .c.inc"), QEMU standard procedure for included C files is to
use *.c.inc.

Besides, since commit 6a0057aa22 ("docs/devel: make a statement
about includes") this is documented in the Coding Style:

  If you do use template header files they should be named with
  the ``.c.inc`` or ``.h.inc`` suffix to make it clear they are
  being included for expansion.

Rename "test_akcipher_keys.inc" as "test_akcipher_keys.c.inc".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>