tgsi: Add STENCIL to text parser.

Fix OpenVG "filter" demo

  Program received signal SIGSEGV, Segmentation fault.
  0xb7153dc9 in str_match_no_case (pcur=0xbfffe564, str=0x0) at
  tgsi/tgsi_text.c:86
  86         while (*str != '\0' && *str == uprcase( *cur )) {

src/gallium/auxiliary/tgsi/tgsi_text.c

@@ -1007,7 +1007,8 @@ static const char *semantic_names[TGSI_SEMANTIC_COUNT] =
    "FACE",
    "EDGEFLAG",
    "PRIM_ID",
-   "INSTANCEID"
+   "INSTANCEID",
+   "STENCIL"
 };
 
 static const char *interpolate_names[TGSI_INTERPOLATE_COUNT] =