[qhull] Remove problematic friendship

For private access within the module, don't use out-of-namespace friendship with
an `extern "C"` function. This doesn't work with drake's namespace vendoring.

Instead, just make the fields public.

Reasoning for not upstreaming this patch: Drake-specific vendoring.

The Qhull license requires us to publish the author, date, and reason
for changes: Author: Jeremy Nimmer; Date: Jun 2, 2022.

--- src/libqhullcpp/QhullQh.h.orig
+++ src/libqhullcpp/QhullQh.h
@@ -58,7 +58,7 @@
 #//!\name Constants
 
 #//!\name Fields
-private:
+public:
     int                 qhull_status;   //!< qh_ERRnone if valid
     std::string         qhull_message;  //!< Returned messages from libqhull_r
     std::ostream *      error_stream;   //!< overrides errorMessage, use appendQhullMessage()
@@ -66,8 +66,9 @@
     double              factor_epsilon; //!< Factor to increase ANGLEround and DISTround for hyperplane equality
     bool                use_output_stream; //!< True if using output_stream
 
+private:
     //! modified by qh_fprintf in QhullUser.cpp
-    friend void         ::qh_fprintf(qhT *qh, FILE *fp, int msgcode, const char *fmt, ... );
+    //friend void         ::qh_fprintf(qhT *qh, FILE *fp, int msgcode, const char *fmt, ... );
 
     static const double default_factor_epsilon;  //!< Default factor_epsilon is 1.0, never updated
 
