Fix Linux build.

Fix Linux build.

We don't need protected visibility because we now have a version script
which hides all symbols except the ones we want to export.

Fixes ld linker errors:
 relocation R_X86_64_PC32 against protected symbol `libEGL_swiftshader' can not be used when making a shared object
 relocation R_X86_64_PC32 against protected symbol `libGLESv2_swiftshader' can not be used when making a shared object

Change-Id: I059c4b03f2523bf8caf08180b02a349d3cf7b2f2
Reviewed-on: https://swiftshader-review.googlesource.com/18708
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>