gdb: fix gnu-nat build

gdb: fix gnu-nat build

When building gnu-nat.c, we get:

      CXX    gnu-nat.o
    gnu-nat.c: In member function 'virtual void gnu_nat_target::create_inferior(const char*, const string&, char**, int)':
    gnu-nat.c:2117:13: error: 'struct inf' has no member named 'target_is_pushed'
     2117 |   if (!inf->target_is_pushed (this))
          |             ^~~~~~~~~~~~~~~~
    gnu-nat.c:2118:10: error: 'struct inf' has no member named 'push_target'
     2118 |     inf->push_target (this);
          |          ^~~~~~~~~~~

This is because of a confusion between the generic `struct inferior`
variable and the gnu-nat-specific `struct inf` variable.  Fix by
referring to `inferior`, not `inf`.

Adjust the comment on top of `struct inf` to clarify the purpose of that
type.

Co-Authored-By: Andrea Monaco <andrea.monaco@autistici.org>
Change-Id: I2fe2f7f6ef61a38d79860fd262b08835c963fc77