tests: add "include-fixed" GCC directory to include directories

On Arch Linux, the GCC version of "limits.h" is in the "include-fixed" instead
of the "include" directory. It needs to be included in the include directories
list, otherwise attempting to compile the test suite fails with the following
error:

In file included from /usr/include/efivar/efivar-dp.h:22,
                 from /usr/include/efivar/efivar.h:238,
                 from include/test.h:51,
                 from shim.h:68,
                 from csv.c:6:
/usr/include/limits.h:124:16: fatal error: limits.h: No such file or directory
  124 | # include_next <limits.h>
      |                ^~~~~~~~~~
compilation terminated.

include/test.mk

@@ -46,6 +46,10 @@ CFLAGS = $(OPTIMIZATIONS) -std=gnu11 \
 	 -DSHIM_UNIT_TEST \
 	 "-DDEFAULT_DEBUG_PRINT_STATE=$(DEBUG_PRINTS)"
 
+# On some systems (e.g. Arch Linux), limits.h is in the "include-fixed" instead
+# of the "include" directory
+CFLAGS += -isystem $(shell $(CC) $(ARCH_CFLAGS) -print-file-name=include-fixed)
+
 export CFLAGS_LTO CFLAGS_GCOV
 
 libefi-test.a :