Silence compilation warning

This fixes the following warnings:

In file included from ass_cache.c:34:0:
ass_cache_template.h:52:0: warning: "BINSTRING" redefined
 #define BINSTRING(member) \
 ^
In file included from ass_cache.h:59:0,
                 from ass_font.h:58,
                 from ass_cache.c:28:
ass_cache_template.h:9:0: note: this is the location of the previous definition
 #define BINSTRING(member) \
 ^
In file included from ass_cache.c:36:0:
ass_cache_template.h:30:0: warning: "BINSTRING" redefined
 #define BINSTRING(member) \
 ^
In file included from ass_cache.c:34:0:
ass_cache_template.h:52:0: note: this is the location of the previous definition
 #define BINSTRING(member) \
 ^
Provide slightly more fine-grained control over style overrides