ustr: fix build issue with GCC5

This patch fixes the following error messages:

ustr-cmp-code-so-dbg.o: In function `ustr_pool_make_subpool':
ustr-cmp-dbg-code.c:(.text+0x0): multiple definition of `ustr_pool_make_subpool'
ustr-b-code-so-dbg.o:ustr-b-dbg-code.c:(.text+0x0): first defined here
ustr-cmp-code-so-dbg.o: In function `ustr_pool_clear':
ustr-cmp-dbg-code.c:(.text+0x44): multiple definition of `ustr_pool_clear'
ustr-b-code-so-dbg.o:ustr-b-dbg-code.c:(.text+0x44): first defined here
ustr-cmp-code-so-dbg.o: In function `ustr_pool_free':
...

That's because the application assumes gnu89 behaviour of "inline" and
"extern inline", but gcc5 defaults to gnu11. So we need to force gnu89
in the compilation flags.

Fixes:
http://autobuild.buildroot.org/results/fde02ae2883a058b98e57ba087466d820ab1a5eb/
http://autobuild.buildroot.org/results/e04849c31e2a2528282ce6cdcf69aa10076d721d/
http://autobuild.buildroot.org/results/7b3295e711fc51907c311adb63f0b7d06a90a7b9/


...and many others.

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>