Add missing header guards.

blur.h

@@ -1,4 +1,8 @@
+#ifndef BLUR_H
+#define BLUR_H
 
 LIQ_PRIVATE void liq_blur(unsigned char *src, unsigned char *tmp, unsigned char *dst, unsigned int width, unsigned int height, unsigned int size);
 LIQ_PRIVATE void liq_max3(unsigned char *src, unsigned char *dst, unsigned int width, unsigned int height);
 LIQ_PRIVATE void liq_min3(unsigned char *src, unsigned char *dst, unsigned int width, unsigned int height);
+
+#endif

mediancut.h

@@ -1,2 +1,6 @@
+#ifndef MEDIANCUT_H
+#define MEDIANCUT_H
 
 LIQ_PRIVATE colormap *mediancut(histogram *hist, unsigned int newcolors, const double target_mse, const double max_mse, void* (*malloc)(size_t), void (*free)(void*));
+
+#endif

nearest.h

@@ -2,7 +2,13 @@
 //  nearest.h
 //  pngquant
 //
+
+#ifndef NEAREST_H
+#define NEAREST_H
+
 struct nearest_map;
 LIQ_PRIVATE struct nearest_map *nearest_init(const colormap *palette);
 LIQ_PRIVATE unsigned int nearest_search(const struct nearest_map *map, const f_pixel *px, const int palette_index_guess, float *diff);
 LIQ_PRIVATE void nearest_free(struct nearest_map *map);
+
+#endif