checkpolicy: Treat invalid characters as an error

Previously the behavior was to warn, discard the character and proceed.
Now the build will halt upon encountering an unexpected character.

Signed-off-by: Daniel Burgener <dburgener@linux.microsoft.com>
Acked-by: James Carter <jwcart2@gmail.com>

checkpolicy/policy_scan.l

@@ -290,7 +290,7 @@ GLBLUB				{ return(GLBLUB); }
 "]" |
 "~" |
 "*"				{ return(yytext[0]); } 
-.                               { yywarn("unrecognized character");}
+.                               { yyerror("unrecognized character");}
 %%
 int yyerror(const char *msg)
 {