Don't add the vo-amrwbenc directory to the include path in the pkg-config file

Require that users of the library include the header as
vo-amrwbenc/enc_if.h.

vo-amrwbenc.pc.in

 prefix=@prefix@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
-includedir=@includedir@/vo-amrwbenc
+includedir=@includedir@
 
 Name: VisualOn AMR-WB encoder
 Description: Adaptive Multi-Rate Wideband speech codec library