Cleanup: move error reporting function declarations to errorhelper.h

Move the declarations of the "report_error()" and "set_error_cb()"
functions and the "verbose" variable to errorhelper.h.
Thus, error-reporting translation units don't have to import the
big dive.h header file.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/libdivecomputer.h

@@ -2,6 +2,7 @@
 #ifndef LIBDIVECOMPUTER_H
 #define LIBDIVECOMPUTER_H
 
+#include <stdint.h>
 
 /* libdivecomputer */
 
@@ -18,12 +19,15 @@
 #define dc_usb_storage_open(stream, context, devname) (DC_STATUS_UNSUPPORTED)
 #endif
 
-#include "dive.h"
-
 #ifdef __cplusplus
 extern "C" {
+#else
+#include <stdbool.h>
 #endif
 
+struct dive;
+struct dive_computer;
+
 typedef struct dc_user_device_t
 {
 	dc_descriptor_t *descriptor;