Fix: add missing #include for 3.8 kernel

Fix: add missing #include for 3.8 kernel

Add the `linux/compat.h` include to fix the following error while
building the modules against a 3.8 kernel for x86-64 kernel:
  error: ‘F_GETLK64’ undeclared here (not in a function)
     ctf_enum_value("F_GETLK64", F_GETLK64)

Also, move the #include directives from the instrumentation headers to
the `lttng-syscalls.h` file.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I334a14d0b40ec77e0ca0aa3b762197de2050a1ae