tools/Makefile: build host tools with -std=gnu99

Parts of the code are using C99 constructs (such as variables declared
inside loops), but also GNU extensions (such as typeof), so using
-std=gnu99 is necessary to build with older versions of gcc that don't
default to building with gnu99.

It fixes the following build failure:

./tools/../lib/crc16.c: In function "crc16_ccitt":
./tools/../lib/crc16.c:70:2: error: "for" loop initial declarations are only allowed in C99 mode
  for (int i = 0;  i < len;  i++)
  ^
./tools/../lib/crc16.c:70:2: note: use option -std=c99 or -std=gnu99 to compile your code

when building the host tools with gcc 4.7.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>