src/Makefile: correcting tabulation to fix verbose build

[ia]

TL; DR: fix tab for USE_VERBOSE_COMPILE=yes make use case.

Hello again. I have a couple of other ideas for further little improvements of src/Makefile, if no one minds. But during experiments, I discovered a very little issue which I'm happy to help to fix first.

Steps to reproduce:

• run a local build with verbose output using supported make variable:
  $ USE_VERBOSE_COMPILE=yes make

What should happen? :

• successfull local build with detailed output

What actually happens? :

• build stops due to error:
  Makefile:327: *** missing separator. Stop.

Root cause:

• a set of spaces placed in the line with error instead of tab

Hence, this microscopic PR commit fixes verbose building.