configure: enable -Wstrict-prototypes unconditionally

I noticed that -Wstrict-prototypes would not get enabled, because the
autoconf test program generates a warning of this type.  Enable it
unconditionally, like we already do for -Wold-style-definition.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I145ceedb977d1d60a094f598ecae56930c742c4c