build: do not indent assignments in Makefile

Indented assignment in a Makefile can be interpreted
as a command in e.g. GNU Make 3.81 which results in the
following error:

```
make: CPPLINT_QUIET: No such file or directory
```

PR-URL: #29623
Reviewed-By: Sam Roberts <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>

Author: joyeecheung

Makefile

@@ -1312,9 +1312,9 @@ else
 endif
 
 ifeq ($(V),1)
-	CPPLINT_QUIET =
+CPPLINT_QUIET =
 else
-	CPPLINT_QUIET = --quiet
+CPPLINT_QUIET = --quiet
 endif
 .PHONY: lint-cpp
 # Lints the C++ code with cpplint.py and check-imports.py.