spi_flash: fix build for Linux target

CONFIG_MMU_PAGE_SIZE is not defined for the Linux target, add a
fallback definition.

components/spi_flash/partition.c

@@ -30,6 +30,13 @@
 #include "esp_private/partition_linux.h"
 #endif
 
+#ifndef CONFIG_IDF_TARGET_LINUX
+#define MMU_PAGE_SIZE CONFIG_MMU_PAGE_SIZE
+#else
+// No relation to the page size on Linux; assume the same value as on ESP32
+#define MMU_PAGE_SIZE 65536
+#endif // CONFIG_MMU_PAGE_SIZE
+
 #ifndef NDEBUG
 // Enable built-in checks in queue.h in debug builds
 #define INVARIANTS
@@ -78,7 +85,7 @@ static esp_err_t load_partitions(void)
     esp_rom_md5_init(&context);
 #endif
 
-    uint32_t partition_align_pg_size = (ESP_PARTITION_TABLE_OFFSET) & ~(CONFIG_MMU_PAGE_SIZE - 1);
+    uint32_t partition_align_pg_size = (ESP_PARTITION_TABLE_OFFSET) & ~(MMU_PAGE_SIZE - 1);
     uint32_t partition_pad = ESP_PARTITION_TABLE_OFFSET - partition_align_pg_size;
 
 #if CONFIG_IDF_TARGET_LINUX