Use typedefs to work around dtrace dropping const qualifiers from probe
arguments when generating Xserver-dtrace.h.   Add new probes.h header to
avoid having to replicate these typedefs in every file with dtrace probes.

Gets rid of these warnings from gcc 4.8:
 getevents.c:1096:9:
  warning: passing argument 6 of '__dtrace_Xserver___input__event' discards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1096:9:
  warning: passing argument 7 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1651:9:
  warning: passing argument 6 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1651:9:
  warning: passing argument 7 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1791:9:
  warning: passing argument 6 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1791:9:
  warning: passing argument 7 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1921:9:
  warning: passing argument 6 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]
 getevents.c:1921:9:
  warning: passing argument 7 of '__dtrace_Xserver___input__event' disards
  'const' qualifier from pointer target type [enabled by default]

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>