Remove duplicated cmake_minimum_required() calls.

OGLCompilersDLL/CMakeLists.txt

-cmake_minimum_required(VERSION 2.8)
-
 set(SOURCES InitializeDll.cpp InitializeDll.h)
 
 add_library(OGLCompiler STATIC ${SOURCES})

SPIRV/CMakeLists.txt

-cmake_minimum_required(VERSION 2.8)
-
 set(SOURCES
     GlslangToSpv.cpp
     InReadableOrder.cpp

StandAlone/CMakeLists.txt

-cmake_minimum_required(VERSION 2.8)
-
 add_library(glslang-default-resource-limits
     ${CMAKE_CURRENT_SOURCE_DIR}/DefaultResourceLimits.cpp
 )

glslang/CMakeLists.txt

-cmake_minimum_required(VERSION 2.8)
-
 if(WIN32)
     add_subdirectory(OSDependent/Windows)
 elseif(UNIX)

glslang/OSDependent/Unix/CMakeLists.txt

-
-
-cmake_minimum_required(VERSION 2.8)
-
 add_library(OSDependent STATIC ossource.cpp ../osinclude.h)
 
 install(TARGETS OSDependent 

glslang/OSDependent/Windows/CMakeLists.txt

-cmake_minimum_required(VERSION 2.8)
-
 set(SOURCES ossource.cpp ../osinclude.h)
 
 add_library(OSDependent STATIC ${SOURCES})

hlsl/CMakeLists.txt

-cmake_minimum_required(VERSION 2.8)
-
 set(SOURCES
     hlslParseHelper.cpp
     hlslScanContext.cpp