meson.build: enable compile time prototype enforcement

Help enforce that file-local functions and variables will be made
static by requiring that exported symbols require a prototype.  This
was done using two flags supported in clang:

   -Wmissing-variable-declarations:
     Enforce that global variables have a prototype declaring them as
     extern preceding.

   -Wmissing-prototypes:
     Enforce that functions which are visible have prototypes
     preceding.

BUG=chromium:990438
TEST=compiles

Change-Id: Ia9dd89bdc7264727da9eb96593c3aaf4e164d2dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/2204961
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
1 file changed