Prevent automake warning about deprecated INCLUDES (#735742)

Automake 1.13 and later generates the following warning when building
GParted from git:

    $ ./autogen.sh
    ...
    checking for automake >= 1.9...
      testing automake-1.13... found 1.13.4
    ...
    Running automake-1.13...
    src/Makefile.am:1: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
    ...

INCLUDES name has been depreciated since automake 1.7.  GParted's
autogen.sh and configure.ac require at least automake 1.9.  Therefore
replace automake variable INCLUDES with AM_CPPFLAGS.

Bug #735742 - automake warns INCLUDES is deprecated