libnfs.pc.in: fix pkg-config --cflags

pkg-config --cflags libnfs returns an error with
this line:

$ pkg-config --cflags libnfs
Package @LIBNFS_PC_REQ_PRIVATE@ was not found in the pkg-config search path.
Perhaps you should add the directory containing `@LIBNFS_PC_REQ_PRIVATE@.pc'
to the PKG_CONFIG_PATH environment variable
Package '@LIBNFS_PC_REQ_PRIVATE@', required by 'libnfs', not found

Signed-off-by: Peter Lieven <pl@kamp.de>