Fix compilation errors

Fixes the following compilation errors:
examples/pci_encoder_read.c:33:19: error: unknown type name ‘useconds_t’; did you mean ‘suseconds_t’?
   33 | extern int usleep(useconds_t usec); // static global access variable conflicts with POSIX function in unistd.h
      |                   ^~~~~~~~~~
      |                   suseconds_t
examples/pci_encoder_read.c: In function ‘main’:
examples/pci_encoder_read.c:171:9: warning: implicit declaration of function ‘usleep’ [-Wimplicit-function-declaration]
  171 |         usleep(delay*1000);             // wait delay ms
      |         ^~~~~~

examples/pci_encoder_read.c:40:23: error: ‘access’ redeclared as different kind of symbol
   40 | static board_access_t access;
      |                       ^~~~~~
In file included from examples/pci_encoder_read.c:29:
/usr/include/unistd.h:287:12: note: previous declaration of ‘access’ was here
  287 | extern int access (const char *__name, int __type) __THROW __nonnull ((1));
      |            ^~~~~~

examples/pci_analog_write.c:33:19: error: unknown type name ‘useconds_t’; did you mean ‘suseconds_t’?
   33 | extern int usleep(useconds_t usec); // static global access variable conflicts with POSIX function in unistd.h
      |                   ^~~~~~~~~~
      |                   suseconds_t

examples/pci_analog_write.c:40:23: error: ‘access’ redeclared as different kind of symbol
   40 | static board_access_t access;
      |                       ^~~~~~
In file included from examples/pci_analog_write.c:29:
/usr/include/unistd.h:287:12: note: previous declaration of ‘access’ was here
  287 | extern int access (const char *__name, int __type) __THROW __nonnull ((1));
      |            ^~~~~~

Signed-off-by: Damian Wrobel <dwrobel@ertelnet.rybnik.pl>