Fix public header include paths

This is a fix for dvdnav failing to build with dvdread as a Meson subproject, caused by missing macros from dvdread's version.h. As dvdnav has its own version.h, the dvdread one is never included. To fix it, the include directives need to use the full path to the other dvdread headers.

[3/29] Compiling C object src/libdvdnav.so.4.4.0.p/vm_vm.c.o
FAILED: src/libdvdnav.so.4.4.0.p/vm_vm.c.o 
cc -Isrc/libdvdnav.so.4.4.0.p -Isrc -I../src -I. -I.. -Isrc/dvdnav -I../src/dvdnav -I../src/vm -Isubprojects/dvdread/src -I../subprojects/dvdread/src -fvisibility=hidden -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=c17 -O2 -g -DHAVE_CONFIG_H -mno-ms-bitfields -Wsign-compare -D_DEFAULT_SOURCE -fPIC -pthread -DDVDNAV_API_EXPORT -MD -MQ src/libdvdnav.so.4.4.0.p/vm_vm.c.o -MF src/libdvdnav.so.4.4.0.p/vm_vm.c.o.d -o src/libdvdnav.so.4.4.0.p/vm_vm.c.o -c ../src/vm/vm.c
../src/vm/vm.c:64:44: error: missing binary operator before token "("
   64 | #if DVDREAD_VERSION >= DVDREAD_VERSION_CODE(6,1,0)
      |                                            ^
../src/vm/vm.c: In function ‘vm_reset’:
../src/vm/vm.c:425:44: error: missing binary operator before token "("
  425 | #if DVDREAD_VERSION >= DVDREAD_VERSION_CODE(6,1,0)
      |                                            ^