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.

main.c

@@ -207,7 +207,7 @@ int main(int argc, char* argv[]) {
   int dumpopts = JV_PRINT_INDENT_FLAGS(2);
   const char* program = 0;
 
-  jq_util_input_state input_state = jq_util_input_init(NULL, NULL); // XXX add err_cb
+  jq_util_input_state *input_state = jq_util_input_init(NULL, NULL); // XXX add err_cb
 
   int further_args_are_files = 0;
   int jq_flags = 0;