Include <stddef.h> for size_t

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@39006 15284696-431f-4ddb-bdfa-cd5b030d7da7

include/gmath.h

@@ -29,6 +29,7 @@
  /* only include if available */
 #include <grass/la.h>
 #endif
+#include <stddef.h>
 
 /* fft.c */
 int fft(int, double *[2], int, int, int);