all.sh: Fix check_headers_in_cpp

When all.sh invokes check_headers_in_cpp, a backup config.h exists. This
causes a stray difference vs cpp_dummy_build.cpp. Fix by only collecting
the *.h files in include/mbedtls.

Change-Id: Ifd415027e856858579a6699538f06fc49c793570
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 039b1b8..1035390 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -295,7 +295,7 @@
 }
 
 check_headers_in_cpp () {
-    ls include/mbedtls >headers.txt
+    ls include/mbedtls | grep "\.h$" >headers.txt
     <programs/test/cpp_dummy_build.cpp sed -n 's/"$//; s!^#include "mbedtls/!!p' |
     sort |
     diff headers.txt -