Fix pg11 make error

[steve-chavez]

Hello!

When trying to do make on pg11 I get the error:

mkdir.c: In function ‘fio_mkdir’:
mkdir.c:42:22: error: ‘FALSE’ undeclared (first use in this function)
   42 |     bool recursive = FALSE;
      |                      ^~~~~
mkdir.c:42:22: note: each undeclared identifier is reported only once for each function it appears in
make: *** [<builtin>: mkdir.o] Error 1

The fix is pretty much the same as postgrespro/imgsmlr#8.

Edit: Just noted this should fix #8.

[csimsek]

Thank you very much.