sysutils/plasma5-powerdevil: Fix build on FreeBSD 12.x

/usr/include/sys/sysctl.h:1117:25: error: unknown type name 'u_int'; did you mean 'uint'?
int sysctl(const int *, u_int, void *, size_t *, const void *, size_t);

PR: 275573
Reported by: Sergey V. Koupreyenko <sergey.koupreyenko@gmail.com>