keep prototype consistant with declaration

Hi Daniel,

Just a cleanup... (I generally compile with -Werror setted)

The commit remove a compilation warning:
src/hsts.c:49: warning: 'should_secure_host' declared inline after being called
src/hsts.c:49: warning: previous declaration of 'should_secure_host' was here

the declaration and the prototype of  `should_secure_host` aren't same (`inline` not present in protype)