Fix absolute reference to gcc include path in gcc-plugin Makefile

Nexmon GCC Plugin points to an absolute path right now:
```
	g++ -std=c++11 -Wall -fno-rtti -Wno-literal-suffix -m64 -fPIC -I/usr/lib/gcc/xtensa-esp32-elf/12.2.0/plugin/include/ -c -o $@ $<
```

This should not be the case. Can we point it to somewhere inside the QCAMON folder?