Inlining can only happen in the same compilation unit where the
function was defined, so there is no sense declaring an inline
function in a header if this function is not defined in this
same header.

This fixes the following gcc-5 warning:

In file included from include/libsigrok/libsigrok.h:1066:0,
                 from src/version.c:21:
include/libsigrok/proto.h:36:20: warning: inline function 'sr_rational_set' declared but never defined
 SR_API inline void sr_rational_set(struct sr_rational *r, uint64_t p, uint64_t q);
                    ^