package: set PKG_CONFIG_SYSROOT_DIR in HOST_MAKE_ENV

A host package that uses AUTORECONF (such as host-libglib2)
may end up running './config.status --recheck'. This will
call the configure script with the environment variables
set in HOST_MAKE_ENV. If PKG_CONFIG_SYSROOT_DIR is missing
this will cause the hardcoded sysroot dir take effect leading
to wrong paths.

See commit 22acade2 which works around this problem by
fiddling with the generated .pc file. This commit becomes
obsolete with this fix.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Cc: Daniel Mack <zonque@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>