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>

package/mongoose/mongoose.mk

@@ -19,7 +19,7 @@ MONGOOSE_CFLAGS += -DNS_ENABLE_SSL -lssl -lcrypto -lz
 endif
 
 define MONGOOSE_BUILD_CMDS
-	$(TARGET_CC) $(@D)/examples/server.c $(@D)/mongoose.c \
+	$(TARGET_CC) $(@D)/examples/server.c $(@D)/mongoose.c -I$(@D) \
 		-o $(@D)/examples/server $(MONGOOSE_CFLAGS) -pthread -ldl
 	$(TARGET_CC) -c $(@D)/mongoose.c $(MONGOOSE_CFLAGS) -o $(@D)/mongoose.o
 	$(TARGET_AR) rcs $(@D)/libmongoose.a $(@D)/mongoose.o