Fixes the following GCC 5 warnings and actual bugs:
```
drivers/unix/net_socket_posix.cpp:562:28: warning: comparison between 'enum IP::Type' and 'enum NetSocket::Type' [-Wenum-compare]
modules/gdscript/gdscript_function.cpp:792:26: warning: comparison of constant '17' with boolean expression is always true [-Wbool-compare]
modules/gdscript/gdscript_function.cpp:792:26: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
modules/gdscript/gdscript_parser.cpp:5082:58: warning: comparison of constant '6' with boolean expression is always false [-Wbool-compare]
modules/gdscript/gdscript_parser.cpp:5082:58: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
modules/mbedtls/stream_peer_mbed_tls.cpp:286:45: warning: comparison between 'enum StreamPeerTCP::Status' and 'enum StreamPeerSSL::Status' [-Wenum-compare]
modules/mbedtls/stream_peer_mbed_tls.cpp:313:45: warning: comparison between 'enum StreamPeerTCP::Status' and 'enum StreamPeerSSL::Status' [-Wenum-compare]
```
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
