Fix #76 add explicit struct typedefs

Author: thomasp85

inst/include/systemfonts.h

@@ -13,13 +13,15 @@ struct FontFeature {
   char feature[4];
   int setting;
 };
+typedef struct FontFeature FontFeature;
 // A structure to pass around a single font with features (used by the C interface)
 struct FontSettings {
   char file[PATH_MAX + 1];
   unsigned int index;
   const FontFeature* features;
   int n_features;
 };
+typedef struct FontSettings FontSettings;
 
 // Get the file and index of a font given by its name, along with italic and
 // bold status. Writes filepath to `path` and returns the index