glsl: Fix Android build

glsl: Fix Android build

The build was broken by the line below, added in commit 4f82fed4.
  s_expression.cpp:26: #include <limits>

Mesa's half of the fix is to add 'external/astl/include' to the include
path. The other half of the fix requires implementing
numeric_limits<float>::infinity() in astl, for which I have patches
submitted upstream for review.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>