Bazel: Move include paths to libraries

Instead of having a global list of -I include paths, have each library
provide a -I for the public/ directory using the includes argument.

Change-Id: I2e321752906920cd94beaaab97b8796af85078e4
diff --git a/pw_preprocessor/BUILD b/pw_preprocessor/BUILD
index be20566..22c433a 100644
--- a/pw_preprocessor/BUILD
+++ b/pw_preprocessor/BUILD
@@ -25,6 +25,7 @@
 pw_cc_library(
     name = "pw_preprocessor",
     hdrs = glob(["public/pw_preprocessor/*.h"]),
+    includes = ["public"],
 )
 
 TESTS = [