Silence macOS (clang) compiler warning

warning: anonymous non-C-compatible type given name for linkage
  purposes by typedef declaration;
  add a tag name here [-Wnon-c-typedef-for-linkage]
  399 | typedef struct {
      |               ^

Weird. This file is compiled as C++ but clang warns about a
  "non-C-compatible type".
Anyway, this commit fixes the warning.

src/fl_boxtype.cxx

@@ -396,7 +396,7 @@ void fl_oxy_round_down_box(int x, int y, int w, int h, Fl_Color col);
 // See also Fl::box_bg() which returns true if bit 1 == zero.
 // /////////////////////////////////////////////////////////////////////
 
-typedef struct {
+typedef struct BOXTABLE {
   Fl_Box_Draw_F *f;
   uchar dx, dy, dw, dh;
   int flags;