Don't hide memmem(3).

Index: xdgmime/meson.build
--- xdgmime/meson.build.orig
+++ xdgmime/meson.build
@@ -5,7 +5,6 @@ project('xdgmime', 'c',
 )
 
 add_project_arguments(
-    '-D_POSIX_C_SOURCE=200809L',
     '-DXDG_PREFIX=xdg_test',
     '-DHAVE_MMAP',
     '-Wduplicated-branches',
@@ -35,4 +34,4 @@ if have_cxx
 endif
 
 subdir('src')
-subdir('fuzzing')
\ No newline at end of file
+subdir('fuzzing')
