CorebootModulePkg: Avoid gcc compile fail caused by unrecognized pragma

Prevent gcc from preprocessing Microsofrt specific  pragma. Otherwise
compile will fail with error: -Werror=unknown-pragma.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Scott Duplichan <scott@notabs.org>
---

Index: CorebootModulePkg/Include/Coreboot.h
===================================================================
--- CorebootModulePkg/Include/Coreboot.h	(revision 17108)
+++ CorebootModulePkg/Include/Coreboot.h	(working copy)
@@ -45,7 +45,9 @@
 #ifndef _COREBOOT_PEI_H_INCLUDED_
 #define _COREBOOT_PEI_H_INCLUDED_
 
+#if defined(_MSC_VER)
 #pragma warning( disable : 4200 )
+#endif
 
 #define DYN_CBMEM_ALIGN_SIZE (4096)