build: renable -Wstrict-prototypes compiler warning

build: renable -Wstrict-prototypes compiler warning

The warning -Wstrict-prototypes was disabled by commit
db9b1df0e47996ff8aaea468a11e1e97f64ee126 .

Enable it again, but avoid warnings for WiMax SDK by explicitly disabling the
compiler warning where needed.

Apparently clang does not produce a warning for -Wstrict-prototypes,
hence we don't need a clang specific #pragma.

Signed-off-by: Thomas Haller <thaller@redhat.com>