tools/ccanlint: test_depends_accurate: catch #include of other ccan C files.

Jon sent a test patch for sha256 which "#include <ccan/str/hex/hex.c>"
without adding str/hex to testdepends.  It "worked" (it wouldn't have
linked with "hex.h"), but ccanlint didn't spot the dependency.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>