Include time header for RTC with modern C++ toolchains

[stdstijn]

Summary

• Include <ctime> explicitly in the RTC implementation, which uses tm, time_t, localtime, and mktime.
• Avoid depending on those declarations arriving through unrelated headers when compiling with newer C++ toolchains.

Verification

• In an isolated Inkplate 6COLOR PlatformIO build using Arduino and the GCC 12.2.0 Xtensa toolchain with -std=gnu++23, the build failed on missing time declarations before this include and completed after adding it.
• Compile-only validation; no on-device tests. This is a focused compatibility fix, not a claim that every library example/board supports all C++23 features.