Do not define IR_EXTERNAL_GDB_ENTRY in ir_php.h

[arnaud-lb]

This is a follow-up of #93.

I think that IR_EXTERNAL_GDB_ENTRY should always be defined for PHP.

I propose to the definition from ir_php.h so that it can always be defined here.

Alternatively it could be defined unconditionally in ir_php.h.

[dstogov]

I have fixed this in ir_php.h.

[arnaud-lb]

Thank you!