tools: Stop re-defining -std= when building tools

tools: Stop re-defining -std= when building tools

While we intentionally set -std=gnu11 for building host tools, and have
for quite some time, we never dropped -std=gnu99 from tools/Makefile.
This resulted in passing -std=gnu11 ... -std=gnu99 when building, and
gnu99 would win.  This in turn would result now in warnings such as:
tools/mkeficapsule.c:25:15: warning: redefinition of typedef 'u32' is a C11 feature [-Wtypedef-redefinition]
typedef __u32 u32;
              ^

Signed-off-by: Tom Rini <trini@konsulko.com>