Work around older compilers

As reported on the list:

I am seeing a similar problem building harfbuzz 0.9.5 with Apple gcc
4.0.1 on OS X 10.5 Leopard:

hb-ot-layout-common-private.hh:406: error: 'struct
OT::CoverageFormat1::Iter' is private
hb-ot-layout-common-private.hh:646: error: within this context
hb-ot-layout-common-private.hh:500: error: 'struct
OT::CoverageFormat2::Iter' is private
hb-ot-layout-common-private.hh:647: error: within this context
make[4]: *** [libharfbuzz_la-hb-ot-layout.lo] Error 1

Also reported as happening with MSVC 2005.

src/hb-ot-layout-common-private.hh

@@ -403,6 +403,8 @@ struct CoverageFormat1
       glyphs->add (glyphArray[i]);
   }
 
+  public:
+  /* Older compilers need this to be public. */
   struct Iter {
     inline void init (const struct CoverageFormat1 &c_) { c = &c_; i = 0; };
     inline bool more (void) { return i < c->glyphArray.len; }
@@ -414,6 +416,7 @@ struct CoverageFormat1
     const struct CoverageFormat1 *c;
     unsigned int i;
   };
+  private:
 
   protected:
   USHORT	coverageFormat;	/* Format identifier--format = 1 */
@@ -497,6 +500,8 @@ struct CoverageFormat2
       rangeRecord[i].add_coverage (glyphs);
   }
 
+  public:
+  /* Older compilers need this to be public. */
   struct Iter {
     inline void init (const CoverageFormat2 &c_) {
       c = &c_;
@@ -522,6 +527,7 @@ struct CoverageFormat2
     const struct CoverageFormat2 *c;
     unsigned int i, j, coverage;
   };
+  private:
 
   protected:
   USHORT	coverageFormat;	/* Format identifier--format = 2 */