Fix RPi make install (#2217)

src/Makefile

@@ -147,7 +147,7 @@ endif
 # RAYLIB_SRC_PATH adjustment for different platforms.
 # If using GNU make, we can get the full path to the top of the tree. Windows? BSD?
 # Required for ldconfig or other tools that do not perform path expansion.
-ifeq ($(PLATFORM),PLATFORM_DESKTOP)
+ifeq ($(PLATFORM), $(filter $(PLATFORM),PLATFORM_DESKTOP PLATFORM_RPI))
     ifeq ($(PLATFORM_OS),LINUX)
         RAYLIB_PREFIX  ?= ..
         RAYLIB_SRC_PATH = $(realpath $(RAYLIB_PREFIX))