kconfig: fix line number in recursive inclusion detection

The error message shows a wrong line number if the 'source' directive
is wrapped to the following line.

[Test Code]

  source \
  "Kconfig"

This results in the following error message:

  Recursive inclusion detected.
  Inclusion path:
    current file : Kconfig
    included from: Kconfig:2

The correct message should be as follows:

  Recursive inclusion detected.
  Inclusion path:
    current file : Kconfig
    included from: Kconfig:1

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>