Force STATIC for internal GZDoom SPIRV library

This makes sure the internal version of this library bundled with the
GZDoom source code is used. This prevents the system from building
GZDoom for dynamic linking with an incompatible external library (see
commit 6fafa29 and
<https://forum.zdoom.org/viewtopic.php?f=2&t=64633>).

libraries/glslang/spirv/CMakeLists.txt

@@ -49,7 +49,7 @@ set(SPVREMAP_HEADERS
     SPVRemapper.h
     doc.h)
 
-add_library(SPIRV ${LIB_TYPE} ${SOURCES} ${HEADERS})
+add_library(SPIRV STATIC ${LIB_TYPE} ${SOURCES} ${HEADERS})
 set_property(TARGET SPIRV PROPERTY FOLDER glslang)
 set_property(TARGET SPIRV PROPERTY POSITION_INDEPENDENT_CODE ON)
 target_include_directories(SPIRV PUBLIC