Fix implicit function declarations

Fixes #8

dspl/src/dft/fft_cmplx.c

@@ -24,6 +24,7 @@
 #include <float.h>
 
 #include "dspl.h"
+#include "dft.h"
 
 
 #ifdef DOXYGEN_ENGLISH

dspl/src/dft/ifft_cmplx.c

@@ -24,6 +24,7 @@
 #include <float.h>
 
 #include "dspl.h"
+#include "dft.h"
 
 
 #ifdef DOXYGEN_ENGLISH

dspl/src/gnuplot/gnuplot_create.c

@@ -18,6 +18,7 @@
 * along with Foobar.    If not, see <http://www.gnu.org/licenses/>.
 */
 #include <stdio.h>
+#include <string.h>
 #include <unistd.h>
 #include "dspl.h"