NaCl docs: Fix PPAPI sample C code

Fix #include of {ppp,ppb}_input_event.h.

Add some missing #includes that the sample code depends on for PP_OK,
PP_EXPORT, NULL and strcmp().

There is no "struct PPP_InputEvent".  There is "struct
PPP_InputEvent_0_1" and the typedef name "PPP_InputEvent".

Fix copy-and-pasted reference to PPB_Instance that should be
PPB_InputEvent.

BUG=none
TEST=tested compiling sample

Review URL: https://codereview.chromium.org/72893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235224 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235224