Undefine O_SIZE macro

O_SIZE is OS/2 extension for open().

This fixes compilation error:

-----
    CC ref-filter.o
In file included from f:/lang/gcc/usr/include/fcntl.h:10,
                 from git-compat-util.h:218,
                 from builtin.h:4,
                 from ref-filter.c:1:
ref-filter.c:206:11: error: expected identifier before numeric constant
  206 |    enum { O_SIZE, O_SIZE_DISK } option;
      |           ^~~~~~
ref-filter.c: In function 'objectsize_atom_parser':
ref-filter.c:338:31: error: 'O_SIZE_DISK' undeclared (first use in this function)
  338 |   atom->u.objectsize.option = O_SIZE_DISK;
      |                               ^~~~~~~~~~~
ref-filter.c:338:31: note: each undeclared identifier is reported only once for each function it appears in
ref-filter.c: In function 'grab_common_values':
ref-filter.c:1094:44: error: 'O_SIZE_DISK' undeclared (first use in this function)
 1094 |    if (used_atom[i].u.objectsize.option == O_SIZE_DISK) {
      |                                            ^~~~~~~~~~~
-----

    modified:   ref-filter.c

Author: komh

ref-filter.c

@@ -160,6 +160,11 @@ enum atom_type {
 	ATOM_REST,
 };
 
+#ifdef __OS2__
+/* O_SIZE is OS/2 extension for open() which is defined in sys/fcntl.h. */
+#undef O_SIZE
+#endif
+
 /*
  * An atom is a valid field atom listed below, possibly prefixed with
  * a "*" to denote deref_tag().