samples: dump_http_server: Change where the .inc file is written

KBuild would write the .inc file to the source directory, this was
changed during the CMake migration because whenever possible it should
be avoided to write files outside of the build directory.

But Makefile.posix assumes that these files are generated in the
source directory so we need to keep generating them there for now.

Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>