include: fix compiler warnings

include: fix compiler warnings

This change has motivation to make -Wall -pedantic compiler options to
spit a little bit less noise, which with luck will increase change to
notice real issues.

Multiple of these:

warning: ISO C forbids braced-groups within expressions [-Wpedantic]

And nearly 300 times reported:

include/c.h:75:41: warning: struct has no named members [-Wpedantic]
 #define UL_BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))

Signed-off-by: Sami Kerola <kerolasa@iki.fi>