build: enable and fix -Wshadow

Shadow names can be prone to bugs.  Enable the warning to avoid them in
the future.  Tested by building and running the unit tests.

Change-Id: Ic3227675f4ff40d266ae2a60c66c894f16e5888e
Signed-off-by: Brad Bishop <bradbish@qti.qualcomm.com>
diff --git a/meson.build b/meson.build
index 4508e20..d7b8132 100644
--- a/meson.build
+++ b/meson.build
@@ -32,6 +32,7 @@
         '-Wlogical-op',
         '-Wunused-parameter',
         '-Wdouble-promotion',
+        '-Wshadow',
         language: 'cpp',
     )
 endif