add debug symbols

It's possible that one day we may have a bug.

Change-Id: I1f3d705844edc022d800ff7edaa9c35a53c541c1
diff --git a/build.ninja b/build.ninja
index da54c03..2527b7d 100644
--- a/build.ninja
+++ b/build.ninja
@@ -2,7 +2,7 @@
 
 clang  = ccache clang -fcolor-diagnostics -Weverything
 gcc    = ccache gcc-7 -fdiagnostics-color -Wall -Wextra
-cflags = -Werror -Os
+cflags = -g -Werror -Os
 
 rule clang
     command = $clang $cflags -MD -MF $out.d -c $in -o $out