gdb: update instance_flags field name in gdb-gdb.py.in

gdb: update instance_flags field name in gdb-gdb.py.in

Commit 314ad88df63c ("Use type_instance_flags more throughout") changed
the name of field type::instance_flags to type::m_instance_flags.  It
however missed changing it in the gdb-gdb.py.in file, which results in
this when trying to use the pretty-printer:

    (top-gdb) p *val.type
    Traceback (most recent call last):
      File "/home/smarchi/build/binutils-gdb/gdb/gdb-gdb.py", line 116, in to_string
        % TypeFlagsPrinter(self.val['instance_flags']))
      File "/home/smarchi/build/binutils-gdb/gdb/gdb-gdb.py", line 76, in __str__
        flag_list = [flag.short_name for flag in TYPE_FLAGS
      File "/home/smarchi/build/binutils-gdb/gdb/gdb-gdb.py", line 77, in <listcomp>
        if self.val & flag.value]
    gdb.error: Argument to arithmetic operation not a number or boolean.
    $7 =

This patch fixes it.

gdb/ChangeLog:

	* gdb-gdb.py.in (class StructTypePrettyPrinter) <to_string>:
	Change instance_flags to m_instance_flags.

Change-Id: Ib5e03c08fe41ca11cd71998f2b1c58052879ce95