tools/iio: Use include/uapi with __EXPORTED_HEADERS__

Use the local uapi headers to keep in sync with "recently" added enum
values like IIO_UVINDEX.

Build tested using:
$ make -C <kernelsrcdir> tools/iio
$ make -C <kernelsrcdir>/tools iio
$ make -C <kernelsrcdir>/tools/iio

This follows a strategy similar to that used by tools/hv, tools/net and
tools/leds among others.

Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

tools/iio/Makefile

 CC = $(CROSS_COMPILE)gcc
-CFLAGS += -Wall -g -D_GNU_SOURCE
+CFLAGS += -Wall -g -D_GNU_SOURCE -D__EXPORTED_HEADERS__ -I../../include/uapi -I../../include
 
 BINDIR=usr/bin
 INSTALL_PROGRAM=install -m 755 -p