The xpc.h file is located in SDK ROOT/usr/include/xpc/ which for me is
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/xpc
I would guess that your base SDK configuration is incorrect. Do you have a valid SDK selected? You might want to change it and then change it back. Sometimes Xcode can be weird about that.
I just downloaded that sample code and put the include into the SMJobBlessHelper.c file and it compiled with no problems (on 10.9.3, Xcode 5.1.1).
Note that there isn't a separate XPC framework that you have to include, it's just part of the standard system libraries.
Also, if you are trying to write an XPC service for privileged operations, you will probably need to do a lot of googling for additional information. Apple's documentation on that is pretty weak. This was helpful to me: http://atnan.com/blog/2012/02/29/modern-privileged-helper-tools-using-smjobbless-plus-xpc
Some of the plist entries and compile settings are difficult to figure out.