edk2-firmware: Respect RECIPE_SYSROOT

This is particularly useful to make sure the compiler uses the recipe
sysroot for headers avoiding host contaminations like:

| opts-GCC.h:42:
| /usr/include/stdint.h:26:10: fatal error: 'bits/libc-header-start.h'
file not found
| #include <bits/libc-header-start.h>

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>