[magma] Standardize on <lib/magma/magma.h> include

According to
https://fuchsia.dev/fuchsia-src/development/languages/c-cpp/naming?hl=en
library headers should be under lib/ and should be included using angle
brackets.

Bug: 124035
Change-Id: Ib11a1cf732ea6b5c88fbc60da9a9aa50d5b77933
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/822807
Reviewed-by: Craig Stout <cstout@google.com>