patch 8.2.0828: Travis: regexp patttern doesn't work everywhere

Problem:    Travis: regexp patttern doesn't work everywhere.
Solution:   Use [:blank:] instead of \b. (Ozaki Kiichi, closes #6146)
diff --git a/ci/config.mk.gcc.sed b/ci/config.mk.gcc.sed
index 3b11569..f46e488 100644
--- a/ci/config.mk.gcc.sed
+++ b/ci/config.mk.gcc.sed
@@ -1 +1 @@
-/^CFLAGS\b/s/$/ -Wno-error=maybe-uninitialized/
+/^CFLAGS[[:blank:]]*=/s/$/ -Wno-error=maybe-uninitialized/