Fix regression parsing gcc preprocessor linemarkers

These are preprocessor statement in the form:

 # linenum filename flags

Closes #1475

Examples/test-suite/preproc_gcc_output.i

@@ -0,0 +1,12 @@
+%module preproc_gcc_output
+// Testcase for Github issue #1475 using the output of gcc -E
+
+// The file below was generated using 'gcc -E header1.h'
+// where header1.h included header2.h
+%include "preproc_gcc_output.h"
+
+%{
+void header1_function_a(int a) {}
+void header2_function(int x) {}
+void header1_function_b(int b) {}
+%}