mongoose: fix build failure

mongoose.h looks missing because we're not building locally, so add
the appropriate -I flag. Fixes:
http://autobuild.buildroot.net/results/701/701711626548bd166cd5bc5669e4761ffed074d1/



[Thomas: change the solution to use -I$(@D) instead of switching to
the build directory.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>