libwebp: Fix typo with InstallDev section

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/libwebp/Makefile

@@ -48,7 +48,7 @@ CONFIGURE_ARGS += \
 
 TARGET_CFLAGS += $(FPIC) -flto
 
-define Package/BuildDev
+define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/include
 	$(INSTALL_DATA) $(PKG_INSTALL_DIR)/usr/include/webp/* $(1)/usr/include/webp/
 	$(INSTALL_DIR) $(1)/usr/lib