Speed up symbolic svg loading

If the svg doesn't use the symbolic style classes, we can avoid
loading it multiple times.

This brought the time for loading system-run-symbolic at 256@2
from 6.8ms down to 2ms.

meson.build

@@ -201,6 +201,7 @@ check_functions = [
   'posix_fallocate',
   'sincos',
   'sincosf',
+  'memmem',
 ]
 
 foreach func : check_functions
@@ -208,6 +209,7 @@ foreach func : check_functions
                      args: '-D_GNU_SOURCE',
                      prefix:
                        '#include <stdlib.h>\n' +
+                       '#include <string.h>\n' +
                        '#include <unistd.h>\n' +
                        '#include <sys/mman.h>\n' +
                        '#include <fcntl.h>\n' +