Fix Kernel build with GCC 10

GCC 10 also causes problems when building the Linux kernel, also because of the change of default behavior -fcommon (see #38) (although compilation happens with arm-linux-gnueabihf-gcc, linking is obviously done by compilation host's gcc toolchain)

During a linking step, it shows `multiple definition of 'yylloc'`.
It can be patched by removing the line
```YYLTYPE yylloc;```
in ` plutosdr-fw/linux/scripts/dtc/dtc-lexer.l`, which is a redundant global declaration (see https://lkml.org/lkml/2020/4/1/1206).