games/gillo: Fix build with Clang 6

src/entity.h:63:21: error: in-class initializer for static data member of type 'const float' requires 'constexpr' specifier [-Wstatic-float-init]

static const float defaultCharge = 10;
                   ^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p478276_s338342/logs/errors/gillo-1.0.b1_10.log