[USB] Reorder "core_pins.h" includes for C++ compatibility

[forderud]

Move the core_pins.h includes in usb.c and usb_seremu.c up to enable building the sources as C++ code. The core_pins.h include needs to be placed above the usb_serial.h and usb_seremu.h includes, since these headers contain C++ code that depends on functions in core_pins.h.

Fixes the following compiler errors if renaming the sources in question from .c to .cpp: In file included from teensy4\usb.cpp:5:

teensy4\usb_seremu.h: In member function 'void usb_seremu_class::begin(long int)': teensy4\usb_seremu.h:68:41: error: 'systick_millis_count' was not declared in this scope
   68 |                 uint32_t millis_begin = systick_millis_count;
      |                                         ^~~~~~~~~~~~~~~~~~~~
teensy4\usb_seremu.h:79:25: error: 'yield' was not declared in this scope
   79 |                         yield();
      |                         ^~~~~
teensy4\usb_seremu.h: In member function 'usb_seremu_class::operator bool()':
teensy4\usb_seremu.h:102:27: error: 'yield' was not declared in this scope
  102 |         operator bool() { yield(); return usb_configuration && usb_seremu_online; }
      |                           ^~~~~
In file included from teensy4\usb_seremu.cpp:32:
teensy4\usb_seremu.h: In member function 'void usb_seremu_class::begin(long int)':
teensy4\usb_seremu.h:68:41: error: 'systick_millis_count' was not declared in this scope
   68 |                 uint32_t millis_begin = systick_millis_count;
      |                                         ^~~~~~~~~~~~~~~~~~~~
teensy4\usb_seremu.h:79:25: error: 'yield' was not declared in this scope
   79 |                         yield();
      |                         ^~~~~
teensy4\usb_seremu.h: In member function 'usb_seremu_class::operator bool()':
teensy4\usb_seremu.h:102:27: error: 'yield' was not declared in this scope
  102 |         operator bool() { yield(); return usb_configuration && usb_seremu_online; }
      |                           ^~~~~