Make jq.h usable from C++

Make jq.h usable from C++

Previously, with clang++:
jq.h:46:37: error: typedef redefinition with different
      types ('struct jq_util_input_state *' vs 'jq_util_input_state')

With g++:
jq.h:46:37: error: conflicting declaration
      ‘typedef struct jq_util_input_state* jq_util_input_state’

This typedef was added to libjq by commit 0d41447 which was
after the 1.4 release, so although it is a public API, this
is not a backcompat break because it has never been in a
release.

Specifying the "*" at all uses of jq_util_input_state is
slightly tedious, but jq_state already works that way, so at
least it will be consistent.