clang-format: set the IncludeBlocks setting to "Preserve"

This was default in clang-format v7, but got changed to "Regroup" in v9.
We prefer the old way for now and setting this explicitly makes it
easier to support both versions.

.clang-format

@@ -3,3 +3,6 @@ BasedOnStyle: Google
 # Google style has a default of 80 lines, but clang-format 7 disregards it in
 # a weird case in modtrezorio-fatfs.h. Setting it explicitly helps.
 ColumnLimit: 80
+# This setting differs between clang-format 7 and 9.
+# Let's set it explicitly to v7 value
+IncludeBlocks: Preserve