Fix regex for unknown preprocessor directives

Signed-off-by: Nathan Sobo <nathan@github.com>

corpus/preprocessor.txt

@@ -133,3 +133,13 @@ int main() {
         (return_statement (number_literal))
         (preproc_else
           (return_statement (number_literal)))))))
+
+====================================
+Unknown preprocessor directives
+====================================
+
+#pragma mark - UIViewController
+
+---
+
+(translation_unit (preproc_call (preproc_directive) (preproc_arg)))

grammar.js

@@ -149,7 +149,7 @@ module.exports = grammar({
       repeat($._compound_statement_item)
     ),
 
-    preproc_directive: $ => /#[ \t]+\a\w*/,
+    preproc_directive: $ => /#[ \t]*\a\w*/,
     preproc_arg: $ => token(prec(-1, repeat1(choice(/./, '\\\n')))),
 
     // Main Grammar