src/core/linux/SDL_threadprio.c: fix build without threads Add an include on SDL_error.h to avoid the following build failure without threads:
/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabihf/8.3.0/../../../../arm-buildroot-linux-uclibcgnueabihf/bin/ld: build/.libs/SDL_threadprio.o: in function `SDL_LinuxSetThreadPriority_REAL': SDL_threadprio.c:(.text+0x0): undefined reference to `SDL_Unsupported'