pw_log_basic: split bazel target

Separate headers and sources into separate targets for bazel for
pw_log_basic. This should allow Pigweed to pass stricter dependency
checks than bazel has.

Bug: 101
Change-Id: I6b60918c41b2753692aa1d48de07c1bda92041ef
diff --git a/pw_log_basic/BUILD b/pw_log_basic/BUILD
index ad7ac07..a16338c 100644
--- a/pw_log_basic/BUILD
+++ b/pw_log_basic/BUILD
@@ -22,10 +22,7 @@
 licenses(["notice"])  # Apache License 2.0
 
 pw_cc_library(
-    name = "pw_log_basic",
-    srcs = [
-        "log_basic.cc",
-    ],
+    name = "headers",
     hdrs = [
         "public/pw_log_basic/log_basic.h",
         "public_overrides/pw_log_backend/log_backend.h",
@@ -35,7 +32,18 @@
         "public_overrides",
     ],
     deps = [
+        "//pw_preprocessor",
+    ],
+)
+
+pw_cc_library(
+    name = "pw_log_basic",
+    srcs = [
+        "log_basic.cc",
+    ],
+    deps = [
         "//pw_log:facade",
+        "//pw_log_basic:headers",
         "//pw_string",
         "//pw_sys_io",
     ],