[harfbuzz.cc] Fix OffsetTable name clash with Mac headers

There's no easy way to undo a "using namespace" in our sources, so by the time
we get to include hb-coretext.cc from harfbuzz.cc, we already have "using namespace OT"
active, which clashes with Mac headers.

Error was:
$ gcc -O3 -Wall -arch i386 -DHAVE_CORETEXT=1 -c harfbuzz.cc -o harfbuzz.o -std=c++11
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/TextUtils.h:288:3: error:
      reference to 'OffsetTable' is ambiguous
  OffsetTable     offsets,
  ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/IntlResources.h:115:41: note:
      candidate found by name lookup is 'OffsetTable'
typedef OffPair                         OffsetTable[3];
                                        ^
./hb-open-file.hh:81:16: note: candidate found by name lookup is 'OT::OffsetTable'
typedef struct OffsetTable
               ^
1 error generated.

src/hb-open-file.hh

@@ -48,7 +48,7 @@ namespace OT {
  */
 
 struct OpenTypeFontFile;
-struct OffsetTable;
+struct OpenTypeOffsetTable;
 struct TTCHeader;
 
 
@@ -78,7 +78,7 @@ typedef struct TableRecord
   DEFINE_SIZE_STATIC (16);
 } OpenTypeTable;
 
-typedef struct OffsetTable
+typedef struct OpenTypeOffsetTable
 {
   friend struct OpenTypeFontFile;
 
@@ -218,7 +218,7 @@ struct TTCHeaderVersion1
   Tag		ttcTag;		/* TrueType Collection ID string: 'ttcf' */
   FixedVersion<>version;	/* Version of the TTC Header (1.0),
 				 * 0x00010000u */
-  LArrayOf<LOffsetTo<OffsetTable>>
+  LArrayOf<LOffsetTo<OpenTypeOffsetTable>>
 		table;		/* Array of offsets to the OffsetTable for each font
 				 * from the beginning of the file */
   public:

src/hb-ot-head-table.hh

@@ -43,7 +43,7 @@ namespace OT {
 
 struct head
 {
-  friend struct OffsetTable;
+  friend struct OpenTypeOffsetTable;
 
   static constexpr hb_tag_t tableTag = HB_OT_TAG_head;